I went with Sakuraba(What can I say? I'm also biased, as Saku is one of my all time favorites). The guy took manly asskickings from Arona & Silva, and didn't quit. He was slammed(Hard, I might add) repeatedly by Rampage, and didn't quit.
Big Nog is a good choice as well. This guy took beatings from Fedor, and didn't quit. A lesser man would've quit, but Nog is just tough as hell. Both Nog's are tough, but Big Nog was getting blasted by Cro Cop & Fedor, and just didn't say "Ok, I give up". Strong chin, and just unbelievable willpower. I don't even think a shotgun would stop either Nogueira brother. Big Nog faced Bob Sapp, took a Ganso Bomb from Sapp, and still went on to armbar him.
Here's one guy who often gets overlooked: Takayama. This guy took 3 manly asskickings from guys who were way better than him. Fujita gave him a long, sickening beating. Frye just tore through the guy, and really made his face look alien-like....well, an alien who was just shot in the face. Schilt gave him another sickening beatdown. Takayama didn't quit, and he showed more heart in the Frye fight, than pretty much anyone not counting Big Nog that year.