As far as I know, the only Leica M series camera bodies with TTL (for flash) are the M6 TTL and the M7. The plain old M6 is not a TTL camera for flash. If you are saying TTL (through the lens just for metering [not flash]) I believe the M6, M6 TTL, and the M7. The M5 and CL had meters, but I do not know if they metered through the lens. I presume the new digital M8 will be TTL for flash as well as meter reading. Will find out hopefully at Photo Plus coming up shortly.
The M5, CL (CLE if you count it), M6, M7, MP, M8 all have TTL metering. Some have TTL flash metering: the M6 TTL (not the 'classic' M6), the CLE (if you count it), and the M7. The M8 has sort-of TTL flash metering via a pre-flash - it fires a short pre-flash without opening the shutter, and measures that.
This is in the rangefinder forum, so I guess that it is restricted to rangefinders.