I've used many Leitz and Leica lenses over many years. I've owned about eight of them and used another...ten, maybe? I can't really recall all of them now, although at one time I could have given you chapter and verse from memory. To that you can add another dozen or so non- Leitz/Leica third party lenses, from makers such as Minolta, Zeiss, Voigtländer, and Konica. But this doesn't make me an expert overall—there are too many lenses for my ~30 to be more than a sampling. However, I'm not unfamiliar with M lenses.
I've generally not used many of the "cost-no-object" statement lenses, with a few exceptions: one picture in my portfolio is a portrait of my friend Oren Grad taken with the now-rare pre-1994 35mm Summilux Aspherical Type 1. It's called either the "Aspherical" [or "ASPHERICAL"—thanks Jeff] because it's written out that way on the lens barrel, as opposed to "ASPH.," or sometimes the "double-aspherical" because it has two such surfaces (the successor lens has one asphere). I believe that it was designed by Walter Watz, that the aspherical surfaces were hand-ground, that Leica had either one or two technicians capable of doing that work, and that either 1k or 4k lenses (I found both numbers on the internet—the former sounds more likely to me) were produced between 1988 and 1994. It's now very rare and considered exclusively a collector's item. There's currently a somewhat worn sample for sale from Alex Yuzbasiyan's Setadel Studios, a reputable reseller, for $25,000. (If you are a Leicaphile and have better information on this lens, please do contribute to the Comments section accessible below.) And I'm currently using the 1994–2010 ASPH. Summilux on my friend Jack's M10 Reporter, compliments of my friend John.
Anyway, please don't ask "what do you think of the [insert name of current high-end Leica lens]?," because I won't know. Except to say it's probably excellent if you don't mind the handling and the cost, because I'm pretty confident that can be said of any of the top current Leica lenses.
Some of my preferences 'n' principles go against the grain. For me personally, handling with an M-lens is more important than optical quality. The advantage of a Leica in the olden days was that it was a small camera that made little noise and attracted little notice. So a small lens with simple controls you can memorize made more sense than any of the "statement" lenses such as the Noctilux. Zone focusing by feel was also important to me when I owned my Leicas—with just a little practice, I learned to set the focus distance without looking through the camera at the rangefinder. The skill worked best between about five feet and infinity, and of course made more sense in good light when you can stop down and exact focus is less critical. Also in my opinion, slower lenses made much more sense than fast ones. Not only are they more likely to be small, but a brighter view isn't necessary for focusing. Of course, it depended on what you were trying to do. But I'd rather have a slow lens on a Leica than a big heavy fast lens that blocked part of the viewfinder view. Other people can think whatever they like.

Collapsible 50mm 7-element Summicron
My two faves
Among the ones I owned or tried, I had two favorites. The first was a sample of the first-generation collapsible 7-element 50mm Summicron that had been custom-multicoated by a previous owner. I don't have a picture of my lens but that's the same kind, above, and here's one on Kent P.'s M4:

The seven-element 50mm Summicron, first available in a collapsible mount and later called the "Rigid" because it was adapted to a non-collapsible mount, was remarkable for two reasons: first, it was the lens that really established Leica as a top lensmaker; Zeiss had a stronger reputation in the '50s and Leica was one of many companies that played catch-up. The 7-element Summicron came along just when magazines were beginning to do lens tests, and it got attention as the best lens ever tested by some big magazines. Its second claim to fame was that it was the preferred lens of Henri Cartier-Bresson for much of his career. He tended to use the latest M camera, whatever it was at the time, but he stuck with his favorite 50mm (I believe, on no other evidence than looking at the pictures, that Leica might have multicoated his lens[es] for him, too). It's often pointed out that Henri carried other lenses, but, regardless of that, almost all of his pictures were made with the 50mm. He only very rarely made a picture with any other focal length. I was told this by Henri's good friend and fellow Magnum photographer Erich Hartmann, who was among the second group to join Magnum after the founders.
All told, I owned five 50mm Summicrons: the collapsible; a rigid; a Dual-Range, which I got cheaply because it was missing the "eyes" and thus of no interest to collectors (best-built lens I ever saw, matched only by a couple of Zeiss Contarex lenses); and two modern ones I bought new. One of those was a German-made 1979–1994 Type 4 and the other was a 1994–2013 Type 5. The last two were pedestrian-but-classic Planar-type designs originally intended to be economical but were very well made. And since construction and QC are half the battle with lenses, both were quite nice.
All my 50mm Summicrons were good lenses, in fact, and I got some exceptional pictures with each of them, but there was a non-Leica, non-M-mount 50mm I liked better than any of them. The collapsible was a bit finicky; it had some spherical aberration and flared a bit more easily than modern lenses (despite the multicoating, which might have been fairly basic and not designed specifically for the lens), and it was poor wide open. Stopped down and used with care it could be almost magical. I never should have sold it, but the way I managed my gear hobby in those days was to sell things to buy something else. That's life.
My other favorite, as many people guessed, was the Minolta 40mm ƒ/2 M-Rokkor for the Minolta CLE. John Kennerdell (whom I haven't heard from in a long time, although he used to write marvelous posts for this site) said he once encountered a Japanese connoisseur who referred to it as "the water lens" because its images were so smooth and liquid. It was the only lens I ever owned that routinely got compliments from non-photographers. (Usually, not to throw any cold water on the enthusiasms of gearheads out there, people just don't care.) It's still available pretty reasonably, but it's not very practical, because of course most M-mount cameras don't have 40mm framelines. The CL (Compact Leica) and later CLE (Compact Leica Electronic, although it was sold only by Minolta) was made for a basic set of 28mm/40mm/90mm.
The, um, king
A special mention must be made of the main lens I bought new when I bought my first M6, the 35mm Summicron-M (1979–1996, called version 4 or the "pre-ASPH" since it was replaced by the first aspherical version). I loved that lens when it was new, and its handling is superb on the M cameras, except for the M8 which changes its effective angle of view. It did have a lot of falloff, but since I edge-burned all my prints anyway I considered that a feature rather than a bug, and it was really only a problem with thin-emulsion films anyway. That's the one I learned how to zone-focus by feel, and the images at first were really wonderful. Although the bokeh at wider apertures and closer in wasn't very good, when stopped down and from middle distances it had remarkably coherent bokeh with beautiful transitions. This isn't how people judge bokeh these days, of course—now it's popular to shoot everything wide open no matter what (see The Dog's Nose). Which to me defeats the whole purpose of buying a good lens in the first place, but oh well. (By the way, I'm the person who originally dubbed that lens "the king of bokeh," and I apologize for that. Who knew the phrase would still be permanently adhering to the lens 25 years later?)
I would probably still be using that lens today if it weren't for one thing: on mine, tiny paint chips started flaking off the aperture blades and collecting on an inner element of the lens—right in the center of the optical path! Eventually this became quite noticeable, both when looking into the lens and also in the pictures, because all the flecks of dirt in the middle of the lens cut contrast. Not enough for normal people to notice but just enough for me to notice, which made it seem like I was being tormented by the Universe. I can't remember now whether I had it cleaned once or just meant to, but basically it just pissed me off—here I had paid a premium for a well-built lens, from a maker that gassed on and on in a superior way about how they were better at lensmaking than anybody else, and the one I sprung for had a flaw caused by basic incompetence. Just made me irritated is all. It's not like other lensmakers don't make lenses with problems from time to time—the original AF-Nikkor 35mm ƒ/2 had persistent problems with lubricant leaking onto the aperture blades, for instance (corrected in the ƒ/2D version). But that's why I sold it. I'll tell you what I wish—I wish I had shot Tri-X in my M6 with that lens from that day to this. But of course if I had, I never would have been a magazine writer about gear, for one thing (you gotta churn gear if you want to write about it).
I also owned or used several other 35mm M-mount lenses, including the M-Hexanon, a Zeiss, and various Voigtländers. And the one in the Konica Hexar AF, which wasn't shabby.
"The" lens
That said, here's my opinion—just one enthusiast's opinion—: a 35mm Summicron makes so much sense on any M camera that it should be considered the default lens. Of course, many famous Leica photographers (such as Garry Winogrand) preferred the 28mm focal length, and many preferred 50mms, and that's cool. But the standard viewfinder is perfect when using the 35mm framelines, the 35mm focal length is a perfect match for the kinds of photography a Leica is best at, and there are too many drawbacks to lenses much faster than ƒ/2—which make most of them more trouble than they're worth, and you just don't really need 'em. If you want to shoot with a Leica, get a 35mm Summicron or a third-party equivalent, and spend your time, effort and money learning how to use it!
Here's what I'd recommend:

Voigtländer Nokton Classic 35mm ƒ/1.4 II
Budget options: Voigtländer makes three options of interest—four if you count the two versions of the fast lens—and you should try 'em all for yourself before deciding. There's the wicked sweet tiny little Color-Skopar 35mm ƒ/2.5 P II (I had one of the originals, and it looked great with fast B&W film), the Ultron Vintage Line 35mm ƒ/2 Aspherical Type II VM, and not one but two versions of the Nokton Classic 35mm ƒ/1.4 II, marked SC and MC, for single-coated and multi-coated. (The reason is that some Japanese lens connoisseurs—who put us to shame—think that single-coated lenses are better for black-and-white film.) The Nokton Classic looks like it would handle the most like the old pre-ASPH Summicron, although I don't know firsthand.
Used option: C'mon, if you're going to use the now-garden-variety (did I just say that?) 35mm Summicron-M ASPH., find a used one! Unless you already have yours. These days new ones are too expensive for what you get. Anybody know how much they cost at B&H when they came out in '96? Anyway that pinpoints the year that Leica got too expensive for Yr. Hmbl. Ed.

Zeiss C Biogon 35mm ƒ/2.8 ZM
Zeiss options: Zeiss makes two nifty M-mount 35mm lenses: the C Biogon T* 35mm ƒ/2.8 ZM and the Biogon T* 35mm ƒ/2 ZM. (Well, they make a third, too, but I wouldn't try that.)

Leica Apo-Summicron-M ƒ/2 ASPH.
Top option: If you really want to go balls-out present-day Leica to the manner or the manor born, go all the way: and that means the Leica 35mm APO-Summicron-M ƒ/2 ASPH. You should buy two, so you can casually say, "of course I have a backup that I keep in the box, in case I misplace this one somewhere."
Vintage option: Find one of those v.4 pre-ASPH's I talked about above. They tend to get wobbly in time, and some of the Canadian ones aren't quite as well built as their German counterparts; and they're lenses that got used, so they can be pretty beat. Caveat emptor. Get explicit return privileges on the one you buy, so you can try it before committing.
Maybe I should put the M-Rokkor 40mm on the M10 and see how it does. Like I haven't got enough to do to make me behinder....
Mike
UPDATE: Look what made Apple News this morning! Cool. (Thanks to Dave Richardson for pointing this out.)

Book o' the Week
Jay Maisel: Light, Color, Gesture. This was suggested by Moose. I got to meet Jay Maisel once. Everybody should meet him in his books, if they haven't already. Might help; cannot hurt. Jay's is some of the most positive, hopeful, and generous picture-taking advice you'll find.
This book link is a portal to Amazon.
B&H has M lenses galore
Original contents copyright 2020 by Michael C. Johnston and/or the bylined author. All Rights Reserved. Links in this post may be to our affiliates; sales through affiliate links may benefit this site. As an Amazon Associate I earn from qualifying purchases.
(To see all the comments, click on the "Comments" link below.)
Featured Comments from:
Josh Hawkins: "To this day I appreciate that you taught me zone focusing/pre-focusing on the M glass. Thanks. I still focus while pulling the camera up to my eye. And personally I’m a fan of the 50mm Dual Range and 35mm Summicron from the pre-aspheric days. Now I need a body to use it on. Maybe after the kids finish college."
Cristoph Hammann: "Re 'Leica 35mm APO-Summicron-M ƒ/2 ASPH. You should buy two, so you can casually say, "of course I have a backup that I keep in the box, in case I misplace this one somewhere."' Had to laugh out loud at that! Devilish advice in view of the current supply situation."
Jeff: "Mike, didn’t you already do enough to pump up the price of the v.4? At least you didn’t call it ‘king of vintage.’ By the way, a fine (Double) ASPHERICAL (in caps) can easily run $35k these days. One Leica forum member has several, which he uses."
Mike replies: Arthur Kramer, the onetime lens guru of Modern Photography magazine and the guy who introduced me to Jay Maisel, had a rare 105mm APO-El-Nikkor enlarging lens cast in lucite that he used on his desk as a paperweight. Not sure that counts as "using" it, though! :-)
hilm: "Not a fan of the 35mm lens, but every once in a while, I read that it is the lens to shoot with on a Leica rangefinder. So I do shoot a few rolls, develop them, and go back to shooting with a 50mm.
"My k-of-b type IV (bought used), developed the same black paint specks in the center axis. Sherri cleaned it up for me. My favorite lens is a pre-asph 50mm Summicron. Also the Dual Range, and I have the matching eyes. But if I had to keep just two lenses, I would keep the DR, because it does so much so well and feels so good, and the 35mm, 'cause you have to have one if you have an M-Leica."
Mike replies: So far, Sherri Krauter and Malcolm Taylor have been mentioned in the comments. Those are some pretty august names among Leica repairpeople! Malcolm was chosen by Leica to service the original Ur-Leica, potentially the most valuable camera in the world. Leica Camera AG will never part with it, of course, but if they did, no doubt it would set an astonishing record at auction—surpassing the 1923 Leica 0-Series no. 122 which holds the current record for World's Most Expensive Camera at $2.97 million. The Ur-Leica, for those who don't know, was Oskar Barnack's prototype of the first Leica that he used himself from 1914. And by the way, Malcolm Taylor has stated that the lens on the Ur-Leica was a 40mm, not a 50mm!