the sharpest 50mm? I'd vote for the Nikkor 50mm F2, circa late 70's. But that's not what you asked for. As far as Leica screw mounts, I have heard the Bessa lens are the best. Modern design, good glass, good coatings etc. Just because it says Leica doesn't mean it's the best. Especially some ancient thing.
Hell, I know a couple of guys in Houston that shoot Leicas and they bought the extreme wide angle for the Bessa r and put them on M-6s. That would be the way I would go. Those lenses are pretty damned sharp.
I use the 15mm voigklander Heliar Asph lens on my Leica M7 and it is a great performer at any price, BUT, it does not come any where near to the Leica Summicron
lenses for performance and colour rendering.