Sledge Bros. because they're fat :3 They also cause small earthquakes with their jumps, which is quite awesome and a rather (if not completely) unique ability for a regular enemy.
The plain, old, regular Hammer Bros.. Yes, the guys who throw hammers. I mean come on, we see many throw fireballs and iceballs, but how many times do you see people throwing hammers. Much more intimidating if you ask me.