#2 Former UFC heavyweight Alistair Overeem
One of the most accomplished heavyweights in MMA history, Alistair Overeem won the Strikeforce heavyweight championship in November 2007 against Paul Buentello. Amidst a series of fights under different banners, he even defended the strap once against Brett Rogers.
In 2011, Overeem joined the UFC amid great fanfare. 'The Demolition Man' dominated Brock Lesnar in his first fight with the promotion but lost three of his next four, leaving him to revive his UFC career. He did just that, winning his next four bouts to book a title date against Stipe Miocic.
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But Miocic knocked him out in the very first round, leaving Overeem to repeatedly try and make his way back up the rankings. The Dutchman never won more than two fights on the trot after that, and parted ways with the UFC earlier this year.
#1 Former UFC welterweight title challenger Nick Diaz
Nick Diaz recently made a return to the UFC after a gap of six years and looked completely out of his depth against Robbie Lawler. The Stockton native's UFC career has been marred by controversy and a highly damaging suspension, but he remains a fan favorite and one of the most entertaining fighters in MMA history.
Diaz has competed under various banners in his MMA career, and he held the Strikeforce welterweight championship between January 2010 and April 2011, defending it three times. He then rejoined the UFC, upon which he beat BJ Penn to earn a title shot against Georges St-Pierre.
An injury forced Diaz into an interim title fight against Carlos Condit, which he lost by unanimous decision. He eventually fought St-Pierre, falling to the Canadian via another unanimous decision.
Diaz is now well past his prime, and it's safe to say that he'll never hold a UFC title.