GSP - The guy looks applicably strong. The muscles on his body look like they all serve a purpose, and he seems to man handle people much bigger than him with ease. The guy has to be one of the stongest relative to his weight.
Fedor - We've all seen the things he's done. He tossed rogers like a doll in their fight...on top of that he almost brute forces his submissions, it's ridiculous.
Carwin - The guy has the build of a bull, he might not be the most ripped looking guy, but he's solid and thick.
Brock - Genetically gifted, the guy is a monster, I want to say he's getting to be the GSP of his weight class, it's only a matter of time. He's still a bit too big IMO, but as for applicable strength the guy is a beast, and I'm sure he's super strong lifting wise as well.
Rampage - He seems to have very natural strength, the guy has a really stocky, but even build.
Those are the strongest in my eyes.