Never really thought about it, interesting question. This may be cheating but most often is probably my ratchet clamps. I seem to be using them at the same time as my other tools. If the clamps don't count, I'd say the domino (500) or the RO90 is the most used.
Least is probably my Carvex. Great tool but it's a sometimes tool based on the type of projects I work on. That being said, the Carvex accessory kit is the least used of all of my festool purchases. Thought I'd use it frequently but have really opened it only a few times in well over a year.