I don't know either, but I can't think of any sharper ones...even from Leica.
About a year ago I bought this lens along with a used Leica M6 body. It was a gift to myself for reaching a goal and I've never regretted the expense one bit.
My primary 35mm camera system is Nikon. Certainly not a second-rate system optically. I've used Nikkor lenses for about 30 years and have never found them lacking.
However, I had occasion to shoot the same subject using the Leica 50mm Summicron and a Nikkor 50mm f1.4 AIS.
I had no intention whatsoever of comparing the two lenses. As it turned out, when I put both transparencies (Provia 100F) on the lightbox, I was able to see a slighly crisper image from the Summicron slide. I was using a 4x Rodenstock loupe.
Frankly, the difference I saw wouldn't have convinced me to spend four times the price of the Nikkor lens for the Summicron. At enlargements up to 8x10 or so, the difference would be negligible...at best.
At that point, curiosity got the best of me and I decided to scan both slides using a Nikon Coolscan V ED dedicated film scanner and compare the results at higher magnifications.
Again, at moderate magnification there was little difference. It was only when blown up to a screen size that would equal about an 11x14 print that I started to see a big difference. At a 16x20 equivilent, it was no contest. The Summicaron simply showed more fine detail. Especially in the corners.
I saw no major difference in color accuracy. Contrast was similar, with the Summicron having a slight advantage.
It was only when I took the magnification up to where it would equal a 20X30 inch print that the Summicron image became unacceptable.
When Leica began making lenses for 35mm format, their people knew that the lens has to be able to extract as much information from the tiny 35mm film as possible. What they produced is capable of rendering incredibly sharp photographs, and at the same time, provide beautiful out-of-focus (bokeh), practically no distortion of any kind, be about 40-50% smaller than a 50mm slr lens, has the right amount of contrast, and no vignetting. The photographs made from this lens looks like something made with a medium format Hasselblad. I kid you not.
The current Leica 50mm f2 Summicron-M lens is optically unchanged from the late 1960s version made in Wetzlar, Germany, though it has more multicoating than its predecessor (I own a Wetzlar-made Summicron which produces excellent images, but it still falls behind the Contax Zeiss 50mm and 45mm Planar lenses with respect to both optical quality and performance. EDITORIAL NOTE: I also own both Zeiss Planar lenses, and have found from past experience, that they are slightly better wide open than my Wetzlar-made 50mm Summicron lens.). It is also a bit bulkier, since it has a built-in lens shade which some users have told me isn't really that good in protecting against flare. I'm sure that this lens is a fine image maker, but those interested in optically better 50mm lenses for Leica M mount rangefinder cameras should considered either the expensive Leica 50mm Summilux-M ASPH which has both a built-in lens hood and focusing tab (The current 50mm Summicron doesn't have a focusing tab.) or the new Zeiss 50mm f2 ZM Planar, which is now nearly a third of the price less than that for a brand new 50mm Summicron-M lens. I hope Leica will eventually upgrade this Summicron lens so that its performance is on par with both the Leica Summilux-M ASPH lens and the Zeiss Planar.