Easy. Top hammer, rivet board.
It's the one most consistently useful and necessary for survival, and, on average, brings the most feelings of satisfaction/relief when put to use. It's also the one hammer that you most often have to "earn" (ie, finesse your way through danger to get to it).
(If you're playing the game correctly, you'll rarely use the bottom hammer on rivets
)
Worst: top hammer on conveyors. That's the "derp" hammer.