#2 Frank Shamrock
For a time, Frank Shamrock, adopted brother of the more famous Ken, was the finest MMA fighter in the world.
Shamrock debuted in UFC in December 1997 at UFC Japan and won the Middleweight Championship (which was later rechristened the Light-Heavyweight belt).
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He then went on to defend the title four times, most notably against serial winner, Jeremy Horn and future Light-Heavyweight legend, Tito Ortiz.
His bout with Ortiz was the 1999 fight of the year as the pair traded blows for four rounds before Shamrock finally earned a submission win after barraging his opponents with punches.
Despite being on top of the UFC world, Shamrock then vacated the title and left the promotion citing a lack of competition. He retired a year later only to make unspectacular returns in smaller promotions in 2003 and 2006.
Despite his impressive resume, Shamrock is not well remembered by fans today, largely due to the fact that his peak came during the dark days of the promotion when they were banned in most US states and their reach to a prime audience was limited.