DKs and Pallies generate more AoE threat, but lack mobility.
Warriors can stay mobile with Thunderclap and have shockwave for AoE threat too (but directional).
Druids only have swipe, which does about as much per use as Thunderclap.
So whoever's best depends on the fight. If threat's an issue and the mobs are standing still, pallies and DKs win. If it's a moving fight (like Sarth adds), Warrior works great, with a druid behind him.