Daggers are lighter and can be used with a different weapon or shield. With a warhammer, you miss, than it sucks. With a dagger, if you miss, you can just swing again. And again. And again. Plus, warhammers are ridiculously cumbersome, while daggers aren't. AND daggers can be used stealthily.
I say warhammers. To support this, I bring up a rather popular video game franchise: Warhammer 40k. Specifically, the game Space Marine. In Space Marine, you play as one of the Ultramarines, the best of the best. The first weapons you get are the Bolt Pistol and the knife. Yet the most powerful weapon in the game is, you guessed it, the Warhammer.