Strictly in terms of who was the most dominant at their peak:
1 Royce Gracie
2 Chuck Liddell
3 Ken Shamrock
4 Mark Coleman
Although there a ton of fighters almost/maybe retired who will take over this list (Hughes, Couture, Cro Cop, Wand, Big Nog, Fedor etc.)
Ken isn't retired yet budy.
"The fat man flies at midnight"