Of course my favorite gadget is the sonic screwdriver. Granted, I’m not sure I’ve ever seen it used as a screwdriver, but it’s still awesome. Does pretty much everything. Except it’s useless on old tech, so it does pretty much everything future-y.
I know I’m probably cheating by not picking a single design for the sonic screwdriver as my favorite gadget, but eh. I think they’re all pretty neat.