I would have to say Fedor Emelianenko, the Russian Wargod. On an episode of sport science, they compared the force of his choke to an anaconda. the results Anaconda 40 lbs of pressure. Fedor 500 lbs of pressure! He was actually caving in the head of the dummy as he choked the living piss out of it.
Bruce Lee and Jet Li are very close as well.
as for fictional characters, have to include Alucard from Hellsing, and any of the Belmont family. To continually beat down Dracula (considering how powerful he is in Castlevania) no matter how many times he refuses to stay dead is something that I just have to solute their badassery. OH, to top it off, they do all this with a whip. They fight vampires with a whip.
A discussion on badass characters just wouldn't be complete without mentioning V. How many other characters can say they almost singlehandedly overthrew a government?
Wolverine, moreso for the fact that he kicks rear in nearly every game I have ever seen him appear in.