If they have the other skills to compliment there wrestling I think that wrestlers are the most dominant opponents. Look at Matt Hueghs Great Wrestler - No Stand up (He has never KO and oppent from the feet in 40 MMA matches) - Above average Grappler (Ask GSP). Tito - Dominated the for 3 years w/ wrestling until Chuck and Randy. And the King Fedor, Sambo champ the other guys dont have much of a chance(ask Cro Cop)
Being a world class wrestler that is confident on his back means that you can control and win rounds with out taking serious damage.
Strikers being the next in line, Strikers however must possess the ablity to stay of the ground. If you can stand back up after you get taken down you have a chance against a wrestler but the way that round scoring goes you have to be able to keep yourself on your feet.
Grappling is dead. All the fighters know are so good a submission defence that you just dont see pure grapplers anymore. BJ Penn being one of the best grapplers around uses his stand up for most of his offence. And only after the fighter is worked over they become subseptable to the submissions.
MMA is really about how good you are at all the desiplines. Knowbody is good unless they can mix it up with every form of fighting.