This is even sillier than Per's, but right now my favorite tool is the Turbo Suction Head that I use on the CT Mini for cleaning my house. My wife's dog sheds 6" long hair that is virtually invisible until it builds up enough to make everything look gray. My upright vacuum can't pick it up, so before I got the turbo head, I had to "rake" the carpet with the Festool floor sweep head, pull the hair out of the brush, and then vacuum the carpet with the standard upright vac. This took several hours just to do the living room. The turbo head can now handle the job in minutes, plus it is light and compact.
Aside from that, the Kapex is the most used tool, and the Systainer Ports and Sorttainers have the greatest impact in keeping my shop organized. You guys would probably lynch me if you found out how many tools I have that have never even been plugged in yet, or have never touched a piece of wood.