I would suggest the best all around 50 when you have none is the black cron from the early 70's. You can find one for usually around or under $400. Its the perfect balance between old and new look, is very sharp and has the leica fingerprint. Its a fantastic fantastic lens. Affordable and not nearly 1/4 the lens of some of the others out there for 4 times the price. Once you get that lens and see what you like and dislike, you can make a decision about what direction to go in for your next 50, I think that lens is a very very balanced place to start.
I would not want a ZM sonnar as my only 50. I used mine exclusively for 8 or 9 months to get the hang of it and while it is wonderful, its intensely specific and has as much of a fingerprint at 1.5 as it does at f8. There is more bad information about this lens on the net than probably any other, propagated mostly by people who have never or will never use one. It is a VERY sharp lens. Not so great with flare. Very well made and reasonable at its price point. Its super complimentary to a cron but to have just one lens, Id pick the cron.
The M hex is an amazing lens, both the 50 and 90, I dont know what would need to happen for me to sell either. Amazing lenses, sharper than anything under $1k and absolutely the best for flare out of any M mount lenses I have used.
The Elmar-M is another favorite, very specific look, not the best choice for shooting women.
I would spend a little money on the cron and then see what you like. You are practically, in use, getting just about every bit out of that $400 cron than you will out of a much more expensive lens. Most of these lenses have different looks, but they all perform, generally speaking, to within a very small percentage of each other even though there is tremendous price differences between them. If you dont KNOW that you need to have a single $1500 lens, I would start out with a cheaper cron and then spend the money on the more expensive lenses when you are positive thats what you need to get.