My oldest bike tool presently is my Park Tools crankset extractor wrench with the dual sized screw-in barrel for standard ISO threading and older French crankset threading (I think they quit making it a long time ago.). Otherwise, it would have been the tiny Rivoli chain breaker tool that I lost (like most if them do), a long time ago, in my earliest biking days in college. Those small flat pedal wrenches also tend to lose themselves quite easily. I went through a few sets of those already, too....