There is no "one tool" I love. What I love is always having the right tool for the job. A couple air compressors, 10 or 12 air nailers (might be 14 or 15, I didn't count), 3 chop saws, a table saw, 5 or 6 cordless drills. a sawsall, a couple jig saws, 2 skill saws (worm drive), a rotohammer, 4 or 5 sanders (belt, disk and pad), a couple of power planes, 3 or 4 electric drills, and pretty much every hand tool you've ever seen.
I'm sure there is a ton of stuff I forgot.
btw, I'm a contractor, been collecting tools for 30 years now.