Most used would be my carry gun, a 5" Series I Kimber. At least 100 rounds every weekend, and its not unusual for there to be more.
Least used would be my AMT that I've been stealing various parts off of, grip bushings, various pins, etc. If only complete firearms count, I guess my least used would be my Kimber Polymer, which, ironicly, has the highest round count of all my guns.