That's a little hard to say - I have a couple of Model Ms I am fond of. The later (Blue label Lexmark and Unicomps) seem to be a bit lighter and faster, but I also have a 18 May 1987 (0131) Old School IBM board that I really like - and it is heavier and "crisper". And a 2001 Unicomp M is a real favorite as well - it feels very "right".
There is some variablility, but any decent condition Model M is better than whatever is in second place! I'm a little puzzled by all of this interest in Cherrys, etc - the Holy Grail is the Model M! Right now I am typing on my newest actual IBM/Lexmark marked board, an M13 dated 12/27/1995 - and it rocks just due to having the Trackpoint - no mouse needed! But when I first got it, I had to "recalibrate" my fingers - I didn't intially like the lighter feel! Almost like you could get ahead of yourself! After using it for a couple of weeks, I found it to be just fine. Those who think the older dated Model M is too "heavy" need to look into the mid 1990's Lexmarks, or go with a Unicomp - I think they will be pleased. Something for everyone! Thinkpad Fan