It really depends on what you mean by "dominant champ," i.e., whether you're referring only to how dominant they've been as champions, or how dominant they've been over their careers. Assuming you mean the latter, I'd probably go with Barao. Although he has one defense, which Pettis and Weidman don't, he never beat the champion. Pettis and Weidman both stopped the champions in the 1st and 2nd rounds, respectively. Barao won his belt via decision over a guy the real champion had already beaten.
This is not to say I don't think Barao is dominant. He is only my choice for least because he's being compared to a bunch of other dominant studs.