my damn flip-around screwdriver that fits in the spark plug socket and doubles as the handle that i found in an old kawasaki dirtbike tool kit.
it was the best tool ever until someone stole it last summer
otherwise i'm pretty picky about keeping my tools organized.
pro tip: get a small rubbermaid tote thing, like 5" x 8" x 5" and put all the tools in it, when you dont have a chance to take them back to the tool box. that way they never get mixed in with project junk or buckets of parts. its easier to organize the tool box all at once, and when you do, you can see right away what tools are missing from the complete set.
also i have good expensive tools in a complete set that i always make sure is together. buying harbor freight shit one at a time and keeping it in a pile is a recepie for disaster.