You asked
What are the alternatives to the leica summicron m 50mm f 2?
The closest sibling is the APO-Summicron-M 50mm f/2 ASPH. It has the same focal length and maximum aperture, and I'd call it small and extremely sharp. The classic Summicron is the cheaper of the two, so go for the APO if sharpness is the priority.
If you just want a fast 50mm for people and low light, and you don't need a Leica M, the 50mm f/1.8 is my pick. It's the cheapest great lens in photography and it's made for almost every mount. See portraits for why it works so well for faces, or lenses for where a fast 50mm fits in a first kit.
Written by Claude from this site's pages.