I voted for the M2. Which I no longer own. Bought it in the mid-1980s, along with an M3 which came my way in 1987 via a private sale. Loved those two cameras, the M3 was in a much better working condition than the M2 but as I'm mostly a wide angle photographer, the '2 lived with a 35/3.6 Summaron LTM so one of the oldies which came with an M adapter. Slides I took with those two Ms even with that Summaron and a 50 2.8 Elmar were among the finest I've ever made, mostly I suspect due to their being an advanced yet a very basic tool and I had to take a minimalist viewpoint and try harder to make images I liked.
In the 1990s I set up my own architectural practice. After 18 months there was an economic recession in Australia and I didn't get a contract for a year. Many of my cameras, the Leica kits and several of my beloved Rollei TLRs, had to be sold to keep the business going. Times eventually got better, but by then I was back into Nikons and I never did find an M as good as the two I had been forced to flog off for ready cash. I still regret those sales.
So for me the M2 and M3 are by far the most iconic Leicas I've owned. I have a iif kit now and I greatly enjoy taking it out and photographing with it - but my Ms were truly special cameras and I still miss them.