I think it's Jamie Varner.
Brown isn't exactly "comeback" more like "breakout" - he wasn't ever a contender, he's now just looking a lot better than he ever has.
Varner on the other hand was a champ in WEC, who absolutely looked like he was spiraling the drain and was never coming back in recent years. Shane Roller is sort of understandable, getting KTFO by Dakota Cochrane is not. He's now back and beating (or competing with in the case of Lauzon) quality UFC 155ers, which most would have thought was impossible.