The M2 has viewfinder frames for 35, 50, and 90 mm. The M2 has always been a journalists, and street shooters camera. Very nice with a 1978 era ("Mandler") 35 Summicron.
The M3; 50, 90, and 135. Fans of the 50mm love the M3, it has a higher magnifcation finder.
The M6 has been made in huge numbers, and is plentiful. Available in a few variations.
The M4 is a classic, but very much a collectable.
The M5 is underrated, but is very big ( being the '70s metered camera )
The M42 is despised by collectors, as is the M4P, but they are superb cameras. And usually bargains.
The best buy is one with steady but not violent use. A Leica, like a Rolex, that was purchased new and put in a cupboard for 30 years is useless: you pay a premium and have to have a complete overhaul to clean out the gunk. But one with use, although it may need reasonable work if it hasn't been serviced in a while, will be a good buy. Like a used car.