Only one? How about some must-haves?
Permagrit set of sanding tools would have to be my fave, where I could live with generics of the others listed below.
Razor Plane (I have several, no real fave, they all work well when adjusted right. My allergies really appreciate them reducing sanding)
Morris Building weights.
Vari-speed dremel.
And... and endless supply of good #11 blades in a good handle.
I have a shop full, but I "think" that's my top building tools list. When I think back to being a kid and building everything with a single edge razor and 2 pieces of sandpaper, (rough, and not so rough) I have to laugh, my how we do get spoiled.
EricV