Obviously Fedor, he is pretty much on the top of everybody's list.
Josh Barnett, The HW division needs alot of help and Barnett would be a serious contender.
Mark Hunt, I couldn't see him winning a belt in the UFC, but he would make for some very exciting fights and would be a big hit with the American fans. Hunt vs Kongo and Hunt vs Hardonk would rule.
Aleks E, Like Barnett would be a real threat in the UFC HW division. Alek's fights are usually fun and the guy is a true badass.
Joachim Hansen, Very exciting fighter and could compete at LW or WW.
Gomi, Alot of interesting fights for Gomi in the LW division. Gomi/Florian, Gomi/huerta, Gomi/Sherk, Gomi/Diaz, Gomi/Stevenson, Gomi/Griffin, Gomi/Guillard and Gomi/Penn 2. Damn, the UFC LW division is stacked!