1). Lesnar - The heavy weight division is extremely deep with contenders right now and I see JDS, Cain, Mir and Carwin all giving him a good fight and possibly defeating him.
2). Machida - Until he shows he can defeat Shogun impressivley he'll stay at my number 2 spot as well. He'd probably be closer to the number one spot if Silva or Lil Nog would actually fight him. If he gets past Shogun however I think he'll drop down the list to a safer spot.
3). GSP - Same reasons as LJ, he can possibly get Serra'd again and if that were to happen Hardy is probably the guy to do it. Do I think he'll lose his title soon? No.
4). Anderson Silva - Maia has (albeit slim) a chance to pull off a wonder sub out of no where. And I think Cahel can prove some problems for Silva in the early rounds with his wrestling, so unless Silva can finish him quickly while it's standing or in the later rounds I think Chael could take a UD.
5). BJ Penn - No way he loses at LW. Vanquishes title if he moves up, or gets stripped due to inactivity at 155. There isn't a fighter at 155 in the UFC LW division I can see even making him blink twice right now.