Ross,
I’ve only used two 35 mm lenses on my M6, the f2 from one generation back and a 40+ year-old Summaron (?) f3.5. The latter I regard as a period piece, for collectors, or to use when an “old look” is desired. I would say there is probably not a “best” lens, just a variety of good options.

The Summicron is the main lens on my M6. I use it a lot, and it’s a great lens. At f stops below about f 4 it is probably the equal of the most up-to-the-minute asph. lens, ie. very, very good. I would not spend the extra $ for a lens faster than f2. It’s not worth the money—to me—and the lens is usually bigger and heavier, but perhaps not for the asph models. Face it, all Leica lenses are great, so you can’t go wrong. It’s just a matter of fine tuning your choice to your individual needs and deciding how much value the extra cost of the asph lens is worth to you.
Cheers,
Joe Stephenson