I am a 50mm guy and shoot 80% of time with them. I am happy to give some recommendations for the best starting 50mm lenses for Leica M that I have used and loved:
1. Leica 50mm Summicron (APO & Non-APO are both incredibly sharp)
2. Leica 50mm Summilux (ASPH or Pre-ASPH for different rendering)
3. Voigtlander Nokton 50mm F1.5 ASPH
4. Zeiss Planar 50mm F2 (Sharp as for a fraction of the price)
5. Zeiss Sonnar 50mm F1.5 (Some with back focus issues but the images are surreal)
Those are the best general purpose 50mm lenses that will cover almost any situation. There are also many exotic 50mm lenses that I have used and reviewed, which you can see more on my website.