Frank is a babyface because he only fights in San Jose, and always fights heels (Baroni, Diaz). He used to be a heel, but no more. Now he's the battle tested babyface, taking on the younger versions of himself.
Nick Diaz is a great heel, but he's killing the gimmick because he keeps telling the media it's all an act. I think being hated is getting to him.
Brock dosen't have that problem. He relishes being booed, he dosen't want to be liked, so he's #1.
Tito turned babyface after he lost to Machida, and he completely stole Machida's thunder, just like Hogan did to Ultimate Warrior after WrestleMania 6. Tito is a brilliant self promoter, he took all the attention off Machida and he ended up getting the standing ovation. Tito can easily go back to being a heel though, no problem.
Don't forget Cyborg. The beast who wants to kill the prom queen.
Top Heels:
1.) Brock
2.) Tito
3.) Nick Diaz
Top babyfaces:
1.) GSP. You cant hate him.
2.) Couture. Captain America
3.) Gina Carano
Heel vs. Babyface is the easiest way to draw, MMA needs more heels. I can promise you will see guys trying to rip off Lesnar's promo style now.