Going to go a bit off the cuff to change things up and say Frank Mir.
Though it wasn't his fault, I would have loved to see 2004-ish Mir if he never destroyed his leg/knee wrecking his motorcycle. I think he could have had a decent run with the title. He just never looked the same when he came back.
I'd argue against Bones. Like someone said, he has A LOT of time left to redeem himself and if anything, he's keeping the miles off himself a bit.
BJ because if he cared about his training, he could have done so much more in an already great career.
Ronda because she drinks Edmond's Kool-Aid and let woman's MMA pass her by in a HURRY. If only Ronda knew her hands weren't as good as Edmond said and went to a REAL camp....