Fedor Emelianenko (HW), Anderson Silva (MW), George St-Pierre (WW), BJ Penn (LW). I think LHW is v. debateable since Shogun vs Machida.

Gegard Mousasi is one too look out for, might rise to the top, he looks pretty unstoppable currently.

With the light, middle, and welterweights each UFC champion of these respective divisions looks far above the competition in their division.