Depends on the difficulty.
On Gold, a well played on host PD build Slayer is the most powerful kit in the game. While some can argue it doesn't have diversity, it really has the only thing you need - make enemies dead faster than anything else. Especially if you equip an Acolyte and Incendiary / Disruptor ammo. Maybe, just maybe the best played Talon Merc would have a chance, but still unlikely.
For Platinum, the TGI & GI are the best at disposing of absurdly buffed armor value enemies, although if on Glacier or White the GT, Vsent or Valkryie are right up there.
From a pure diversity point of view, the HE is pretty awesome. QME is awesome too, but I find the playstyle somewhat boring.
And kudos to all the posters chiming in re the HA, it is one of my favorite characters. A bit of a stretch to call it very diverse, but certainly satisfying to play.
Considering all that, give me a Drell ...