#1. Goldberg - Usually refuses to turn heel in WWE
Many WWE fans love trashing Goldberg, but they can't deny the true drawing power he has. Goldberg is one of the few characters in WWE that has a "larger than life" feeling, and it's not surprising that Vince McMahon keeps breaking out his checkbook for him to wrestle for a few minutes.
Goldberg has gotten booed several times from WWE fans, and a heel run would make a lot of sense if he keeps returning annually the way he does. When asked about a heel turn, Goldberg has always shut down the idea - stating that he wants his persona to be like a superhero to kids.
He's well past his prime, and since he occasionally returns, a heel turn for him would be massive. We've never seen a heel Goldberg in WWE, and if he turns, it could result in his best run to date.
One thing is for sure - WWE fans will be far more excited to see a heel Goldberg than a babyface one.