A good night of sleep and the mind-meld with the maniac computer is broken.
A couple of factors are important. When I worked at a camera shop and a customer asked what camera to buy, the two questions I asked where what kind of pictures do you like to take and how much do you want to spend. The first does not apply here, the second does. The questions here are "what is more important to you on a camera: Meter; ease-of-use; quality- especially the viewfinder; cosmetic condition" and "What Lens focal length do you use most often; wide-angle or normal/Telephoto?"
I use fast normals and portrait length telephoto's, and I am comfortable using a handheld meter. For me the M3 is an easy choice. I got a great deal on an M2, spent a lot of time waiting for it to get parts on a repair, and it is a fine camera for the times when I prefer a 35mm focal length lens. But I normally keep a 50 on it, and do not use it as much as the M3. I bought a spare take-up spool for the M3/M2, and keep it pre-threaded in the bag. This makes for drop in loading.
Many people prefer the slotted take-up spool of the M4 and later cameras. I am used to Knob rewind from the Retina's, but many people like crank-rewind. If these appeal to you, the original M4 has the no-expense-spared viewfinder equal to the M2 and MP. It is also the most expensive of the "original" Leica M's.
The M4-2 and M4P are basically M6's without meters. They introduced cost-cutting measures taken in the 1970's to reduce manufacture time and cost. The most annoying problem was a new design for the viewfinder that introduced the "RF Patch Flare" problem that stayed on until the M7 and MP.
The M6 adds the TTL meter. If you want a built in meter, and do not want to spend the money on a new camera, it is your only "Real" choice. The M5 also offers a meter, but expect it to require a rebuild and probably a new CDS cell. For ~$1,000 the M6 is a lot less than an MP or M7.
So Okay, how much do you want to spend, what focal length do you use most, and when can I expect you to join the M3 club -I mean do you need a meter?-