Difficult to do a Top 10 Greatest of all time.
The list with some explanations so you emotional fucks (4th time) don't start arguing.
1. Fedor Emelianenko
2. Lyoto Machida (Boring or not, the guy can adapt to his opponents style and pick him apart)
3. Antonio Rodrigo Nogueira
4. Anderson Silva
5 and 6: BJ Penn & GSP (depends if you want to grab on Penns or GSP's nuts more)
the rest i'm not sure, but should include:
Dan Henderson (i thought he beat Rampage)
and Gegard Mousasi