Awww... don't make me choose!
My favorite tool of all time had to be a big, rubber handled phillips head screwdriver I had. Standard size for computer screws, of course. Could get pretty much any screw in or out with ease - even if it'd been cross threaded. Could get fantastic torque out of it, and use it all day comfortably (important, as I was a dogsbody computer shop tech at the time). I named it "Big Blackie". I loved that screwdriver - it was my #1 offsider for years. Then my idiot ex lost it, and I was furious.
Honourable mentions would be my big torque wrench, soldering iron, digital callipers, 1 metre long screwdrivers, Dremel (with "Flex Shaft" attachment!
) and multi tool (pliers, screwdrivers, knife, LED torch, bottle opener - very small and useful).
Said multi tool has saved my ass a few times - the most memorable when my dizzy cap worked itself loose on the highway at 2am (I must have forgotten to tighten it properly - I had just finished a head gasket change on it the day before).
Not as cool - but I love the simplicity of my "magnet on a stick" and it's cousin, "magnetic bowl".
The former is great for picking up bolts you've dropped into awkward places (say, into a full drip tray). The latter is good for storing bolts you're about to use again later (they stick to the base, and the bowl itself sticks to metal surfaces). Cheap, simple, and very useful.
