1. Machida
2. Anderson Silva
3. GSP
4. Fedor
5. BJ Penn
6. Torres
7. Rampage
8. Mir
9. Barnett
10. Shogun - can't forget about the Pride days, keep in mind the long layoff + injuries when first entering the UFC. He's done too much in Pride and is still far too talented to wipe this guy off of the top 10 list.
This is based off of resume + overall skill and ability. When you take those two things into consideration how is Machida not #1 with wins over Bonnar, Franklin, Tiger White, Soko, Tito, Thiago, and now Evans. Also, take into consideration that he has dominated everyone that he has faced in the UFC thus far and has yet to be rocked by anyone. He takes the least amount of damage out of anyone that there is in the sport. He's also the only guy where you really cannot even sit down and say "that guy can beat him", there's not a fighter in the sport that matches up favorably to Machida and the same cannot be said for anyone else. For that, he's #1.
Silva at #2 because he's undefeated in the UFC and also one of the most versatile and talented guys that there is. When it comes to savy and skill the only other fighter that even compares would be Machida. His resume is also impressive and he has cleaned out his division.
I have GSP at #3 for now, but I can quickly see him climbing to #2 and bumping Silva down one slot. He has had a couple setbacks, but right now he's poised to clean out his division as well and is just as skilled, in his own right, as Anderson Silva is.
Fedor, what more can I say about Fedor, anyone that posted at MM knows how I feel about him. I have a ton of respect for the guy, his resume is impressive, but I just don't see him as one of the most talented fighters that there is. He's a freak of nature, but he's not as well rounded or technical, at least standing, as the guys mentioned above.
The rest of the list is debatable, but that's how I have em right now.