Until someone beats Lesnar, gotta go with him. Yeah yeah, I know that Mir won their first fight, but he didn't really beat him. It was a case of an inexperienced fighter losing focus while beating someone he was better than and giving up a stupid submission, something he clearly fixed in their second fight.
I still think Carwin is the biggest threat to Lesnar, the rematch will be epic if Carwin improves his cardio.
Having said that, you could run that tournament through six times and get six different winners and I wouldn't be the least bit surprised.