At first I didn't know who I was going to pick, then I thought about it for 10 seconds and decided the pretty easy choice was Barnett.
Standing, Barnett is harder to hit, has better technique, has more tools, is much tougher, and has an equally good chin.
Barnett also looks to be improving, he didn't look as flabby in his last fight as he has looked alot in the past. Brock might be able to take him down, but Barnett is game anywhere on the ground and has better cardio from there than he does on his feet. He is also good at leg locks and sweeps/getting back to his feet. It might be hard to do early with a beast like Lesnar, but he will make him work nonetheless, just like Cain did, and we saw how tired Lesnar was after only about 2 minutes into that fight.
I am not convinced at this point that Brock would be able to get an early tko over Barnett, he's to experienced, to tough, to well rounded, and to close to Brock's size for that to happen. Barnett would chip and grind away at Lesnar to a late 2nd or early 3rd round sub or tko, whichever he chooses.