#11 X-acto - definitely the most used item in the toolbox: putty spreader, scriber, seam scraper, ruler, drill, paint stirrer, nail clippers, chip-bag opener, and sometimes even a knife. All-in-one handy device.
For 'fun-factor', I'd have to put my airbrush near the top, although I don't have alot of fun cleaning it.
M.