A little late to the party but...
I have a lot of "favorite" bike tools I've gathered over the past 35+ years: Campy dropout tools, Campy 5mm x 8mm T-wrenches, my adjustable seat tube reamer that I bought at a surplus store for $5 back in 1975, third hands, cable cutters and so on.
The tool that I "appreciate" the most though is the VAR #30 BB fixed cup tool (Campy, Hozan too). I don't use it very often these days but back when we were doing 2-3 BB overhauls a day at the shop where I worked it was a life saver.
http://www.sheldonbrown.com/
I just used it twice in the past two weeks.
Chas. Colerich
Oakland, CA USA