My favorite fighters in the UFC are:
1) Rich Franklin
2) Georges St Pierre/Anderson Silva
4) Mauricio Rua
5) Quinton Jackson/Dan Henderson
It's hard to pick just one because there're many talanted and exciting fighters out there. But #1 is Rich Franklin. I marvel at his ability to recover mentally (he lost to Spider twice but never was broken mentally, the other famous fighters get broken after losing: Liddell, Hughes, Tim Sylvia, Arlovski...) + he's a very talanted fighter + his fights are exciting + he's just a good person (judging by his interviews) + I believe he's still in his prime...