I'd have to say:

Rickson Gracie- though kind of padded, he had an undefeated MMA record and was a dominant force back in the day.

Marco Ruas- he never really got a chance to shine and retired too early, but was one of the first well rounded fighters in MMA.

Bas Rutten- Pancrase champ, UFC champ, hard hitter with an unorthodox submission game.

Chuck Liddell- unquestionably one of the most high profile fighters of modern MMA, he was one of the most dominant LHW fighters in the world.

Igor Vovchanchyn- one of the hardest hitters of all MMA, his career ended when his legendary fists became too damaged for him to continue