My first (used, early Wetzlar) M6 had uneven, streaky exposure at 1/1000s, ruining negatives. Two rolls after it was adjusted by a pro repair shop, it was doing it again. I had another used M6, a TTL that was perfectly functional. I've had wonderful results from my two M7s, with no electrical or mechanical trouble. This isn't a scientific answer, though, just anecdotes. But I doubt Leica would tell you which camera is the most reliable. Newer cameras may have the longest working lives, but a good used camera may be fine for 15-20 years. Unfortunately, it's impossible to predict whether a particular used or new body will be trouble free.
My advice, go to a shop where you can look at M6s, M7s and MPs and buy the one you feel most comfortable using from an ergonomic point of view.