In no particular order:
-Junior Dos Santos: The ass whipping he caught from Cain the first time would be enough for this list, but he came back and soldiered his way through a second brutal beat down. Factor in his wars with Hunto, Carwin, and yes even Nelson....that's a lot of stopping power for one chin to stand up to.
-Carlos Condit: The Natural Born Killer thrives in deep water. He performs better in all hell breaks loose firefights that maintain a high pace and a high volume of violence. Condit is one of, if not the best in the world when it comes to downright fucking ugly brawls.
EDIT: I wanted really badly to include Edgar....but the topic is tough guys. Frankie whines and bitches a bit too much to be considered a tough guy. Js.
-Roy Nelson: Deal with it. Roy has had his ass beat from pillar to post more times than I,can recall, but he never stop fighting even when he's getting dominated. Nelson has a kinda heart that you can't teach, he's just a stubborn motherfucker who won't go away. All bullshit aside, the dude is tough as nails.