Igor Vovchanchyn. In the late 1990s and early 2000s he was one of the most fearful strikers in the game. He hasn't fought in 2 years and no one quite seems to know where he is.
He's always been an exciting fighter, so for newer fans of MMA, he is a definitely a fighter you should familiarize yourself with. He's fought the best of the best and made it a contest virtually everytime.
His destruction of early-UFC-era bad boy Paul Varlens is nothing short of spectacular.