Thomas and Butler by a mile.
Thomas and Butler both impact the game in multiple ways. Thoms with his scoring and play-making, Butler with his scoring, defense and play-making. It takes a special offensive talent to be able to score 20 PPG against NBA defense on a nightly basis - both of those guys are capable of that, and both are actually relatively efficient on offense too. Finally both guys are capable of creating match-up nightmares - Butler with his size, Thomas with his speed.
Westbrook is an absolute beast, but Waiters is pretty much Marcus Thornton 2.0 - he's a volume chucker who contributes very little beyond scoring, and who is incredibly hot/cold as well.
You can't really depend on Waiters to carry the load when Westbrook is out or has a bad game. However if Thomas it out you could easily depend on Butler to step up and carry the load as the go-to guy (and vice versa)/.
So to me, this one is a no brainer - Thomas and Butler all the way.