Based on skill and accomplishment, I wouldn't have Royce in there at all. He was one-dimensional and didn't really fight too many good guys. His win over Ken in UFC 1 is probably his biggest win. I also think GSP has fought better opposition than Hughes, and is certainly more well-rounded. Liddell, again based on level of opposition and the length of his reign, should be ranked below Silva, who went undefeated for AGES in Pride, including three wins against Sakuraba, considered at the time of the first match to be the pound-for-pound best in MMA. So... here's my list:
1) Fedor. He basically has only one legit loss and has faced all sorts of opponents, including Olympic medalists, K-1 kickboxing champs, guys who've held belts in other MMA organizations (including the UFC and Pancrase), ground-and-pound specialists, submission wrestling champs, counter-strikers, brawlers, and giants. He's beaten them all, most in decissive fashion. He's been the top dog in the HW division for years and effectively cleaned out the Pride HW division when it held the best fighters in the world.
2) GSP. Great athleticism, probably the most well-rounded skills in MMA. Hasn't lost a round in years. His two losses have essentially come from the fact that his head wasn't in the game. He's effectively cleaned out the WW division, one of the most packed divisions in MMA.
3) BJ Penn. Probably the most natural talent of any fighter I've ever seen. He's fought upwards of Light Heavyweight, and won as high up as Middleweight. His losses have come from the best of all time. Very well-rounded skills.
4) Antonio Rodrigo Nogueira. Had Fedor never been born, he would have been the best HW fighter in the world for almost a decade. In his prime, he was practically indestructable, and had avenged losses to everyone except Fedor. His top-level ground game, combined with solid boxing and takedowns, made him a force to be reconned with.
5) Bas Rutten. Great striker who learned the submission game. After losing some early matches to some great guys, he went on an undefeated streak. Beat some of the best fighters of his era, including Frank Shamrock, Mas. Funaki, Kevin Randleman, and "TK" Kohsaka.
6) Anderson Silva. Cleaned out the UFC's MW division, though not without some controversy. Arguably the most skilled striker to have sustained success at the top of the MMA world. It could be argued, though, based on a few of his fights that he's not living up to his full, destructive potential, and the few times he's been put on the ground, he still shows a weakness there.
7) Wanderlei Silva. Rulled the Pride "middleweight" (200lbs) division with an iron fist. And iron knees, and iron shins, etc. While not the most skilled fighter, he did effectively clean out his division, went for years without a loss, and had no problem stepping up against larger fighters (Mark Hunt, Mirko Crocop).
8) Dan Henderson. One of the toughest guys in the sport. Fought HW all the way down to MW, and with few exceptions, always gave his opponent a tough fight. Olympic-level wrestling combined with powerhouse boxing made him skilled enough to dismantle most opponents.
9) Matt Hughes. Was the unquestioned king of the WW division, UFC and beyond, for years. While his striking was nothing to write home about, his takedowns, ground-and-pound, top control, and even submissions were excellent.
10) Kazushi Sakuraba. You don't beat four members of the Gracie family and not make the list. He was essentially a MW, some would say WW fighter, competing against heavyweights and LHWs. His grappling was unparalleled, his takedowns dropped all but the best, and his striking, while rudimentary, was creative and came with powerful kicks. Much like Anderson Silva is today, which Saku had his opponent in his world, he'd toy with them, which means a whole lot when "his world" is the ground/submission game and "his opponent" was often a BJJ black belt.