Shogun and Big Nog were champions, at least, after so many wars at PRIDE i think it's understandable that once champs they weren't dominant. (and Big Nog would've beat the shit of Lesnar and then submitted him, Mir was a terrible match up)
Melvin Guillard was always the monster no one wanted to strike with, but never improved his grappling game, i wonder how far he could have gone with Lawler/Aldo TDD level.
Tim Kennedy and Uriah Hall also comes to mind, 2 guys that looked promising while fighting outside, Tim obviously being a former contender at SF having a good fight with Rockhold, and ends with a 3-2 record at UFC losing to a welterweight-sized chicano. Hall and his .500 record was also unexpected.