It's gotta be Couture. He's the obvious choice, which might be why a fair bit of money will go on Lesnar (Yeah, that sounds like a contradiction, but we all know how predictions go in the UFC). Brock has had a pretty decent, promising start, and Couture "MIGHT" be rusty - on the other hand, you have take into account:
1) Randy = badass - Lesnar knows this, and will possibly shit a brick - or just be nervous
2) Experience - total opposite ends of the spectrum, in terms of age and experience.
I think some die-hard UFC fans still look at Lesnar as a bit of a joke, the old WWE faker, but a lot of people, myself included, have been pleasantly surprised by his performances so far.
It'll be an awesome fight, I think - and not one that'll be ended by TKO/Submission. My prediction = Randy by split/unanimous decision? Ok unanimous decision. lol. I can't wait.