Depends on what Best means to you, tbh. Obviously the most over... err... under heels are Cole and Vicki. But as far as pure character, work, and impact in their storylines, I'll go with Cody, Truth and Miz.
Cody has been washed down this past month and 1/2 tbh, but his character itself is still brilliant and he's still generating an incredible performance with it tbh... he hasn't staled yet, and quite honestly, I'm hoping it pays off when this Christian/Orton feud is over and he can be thrown into something high-profile, which, quite honestly, they probably should have done with him coming off a win over Rey at Mania.
Truth has just been pure gold. I've said it several times before, I think he's truly the funniest character on WWE programing in nearly a decade, I've been thoroughly impressed with how well he's handled this gimmick and some of the unexpected twists (like his face-to-face with Hunter when he called Trips crazy for talking to invisible people, lmao), and the conspiracy shit is solid... it's not the freshest gimmick, but it's being done in a very unique and solid way.
Miz is still the premier heel wrestler on the roster, and I'm really hoping they pick something up for him. I will be honest however, and say I'm glad they're giving him some time away from the belt so that he doesn't get the Cena-effect where people tire of seeing him in and out as champ... and he WAS in the finals for the strap anyways... so it's not like they're diminished him in any way. Also, the heat he generated in his promo on Raw after attacking Rey was... awwwwwwesooooooooooome!