On influnce, their time period of fighting and overall greatness.
1.)Royce Gracie - he gets alot of flack from most "smart" fans or Sakuraba marks, but he made mma in the us. Royce should that technique and skill are essiential to combat fighting, and won three of the first 4 UFCs. There would be no UFC if not for the Gracie family.
2.)Randy Couture - His record might not enforce this to new mma fans, but Randy always fought the best competition available to him. Nobody has won more titles, let alone in different weight classes. Nobdody adapted or changed like Couture (see Coleman I guess). And Nobody represented the UFC better or made mma look like a sport. Oh yeah he's still competing and is the heavyweight champion at 43.
3.)Ken Shamrock/Tito Ortiz - Why? Sure not the best ever to compete in the ufc but these guys made the show. UFC 40 was the first show in the states to feature betting lines and captured a whole new audience. Their feud was huge and I hate to say it, Ken's time in the wwf helped "Ultimate fighting" get into the mainstream. Both were also the best during their time period or near the best. Ken or Royce, Tito/Frank Shamrock or now Chuck Lidell. Ken also brought submission and shoot skills to the ufc. Remember "easier, because they don't know submissions"?
4.) Chuck Lidell - Much like Tito/Ken, he's the star of the UFC now. Any card with Chuck Lidell sells, even against Sobral (sorry
). He's their biggest star and his fued with Couture and the subsequent reality show launced the UFC into the mainstream even more than Ken/Tito. Chuck is also considered the #1 fighter in his division and is the most exciting for casual fans to watch.
5.) Matt Hughes - First off Hughes showed that smaller fighters can sell a card. His legacy has made stars out of BJ Penn and GSP. He was the top of the food chain in the UFC's best division, and defended his title more than anyone before him.
Coleman - invented GnP to a degree, first hw champ
Frank Shamrock - As Damone mentioned earlier one of the first legit champs in the ufc and was the total package at the time.
Pat Miletich - First ww champ and a huge influence now as a trainer of several top fighters in the ufc
BJ Penn - he's BJ Penn, one of the most skilled fighters to ever enter the octagon
Rich Franklin - don't start. Rich was a great champ and may well be again. He showed everyone that an ordinary (sort of) guy can be a cage fighter, and represented the ufc very well in return. Leaving a job as a math teacher to be an mixed martial artist is beyond awesome
Phil Baroni - he's the man
Jo Son - I mean anyone who can take those shots in the pills.....
Andrei Arlovski - shows metrosexuals everywhere they can look tough or at least not fruity at some point in their lives