I've got to go with BJ Penn in his prime, at 155.
You could easily argue he had the best boxing, takedown defense, and BJJ in the game, all at the same time. He also has very under-rated takedowns. His only weakness was kicks, he never was a kicker. But he didn't need to be.
Others:
Fedor
Aldo
Bas (though he had by far the weakest competition, he still had a very broad skillset)
I'm sorry, in his 'prime'? at '155'?
You mean when he went to 155 after losing to GSP and getting punched in the face 50 times by Matt Hughes? You consider this guy to be the best all around guy of all time?
Yea he collected some wins at 155 then when he came back to 170 (in his prime), GSP beat the living shit out of him. GSP beat him so bad, Mama Penn was taking it to court.
Then he picked up a couple more wins at 155 before Edgar schooled him. Yea, the first fight was close but in the 2nd fight, Edgar dominated BJ Penn both on the feet, and in grappling. And Frankie's a small guy.
BJ then went on to get beat by almost everyone after that... including getting severely out grappled by Jon Fitch, outboxed by Nick Diaz, and destroyed by Rory.
Don't get me wrong, I think BJ Penn is one of the better fighters during his 'prime', though I'm not sure what that means as he's only 34 now... but no way is he even top10 all time.
I think speaking in terms of all-around fighter, it has to be GSP. Background in striking, best MMA wrestler ever, black belt in bjj, phenomenal conditioning, good game plans, and basically the most impressive reigning career (now that Anderson lost his title).
Fedor was insanely good all around as well but probably not as polished in his striking as GSP. Fedor had insane boxing, but he wasn't a kicker. GSP's striking is just near perfect.