Hard to say I have a "favorite" tool. It depends on the job and how important a certain tool may be to making that job easier.
For example, I have a set of well used 3/8" dr Snap-On deep flex sockets. They PERFECT for getting at the LCA bracket nut on the inside frame rail of a Spitfire. A deep socket is too long as the ratchet hits the oil pan and a shallow socket isn't deep enough. Having that socket turned a 1/2 hr of manually turning a wrench in the frame rail into a 30 second job. That said, the set is otherwise rarely used. But for those 30 seconds, that socket is my favorite tool in the world.
If I had to choose one "favorite" tool, it would probably be a Japanese knock-off of a Snap-On 3/8 dr ratchet I got from Pep Boys 20+ years ago. I keep talking about replacing it (I'd like a ratchet with finer teeth), but it still works.