#1 George St. Pierre (Middleweight Championship, 33 days)
George St. Pierre won the Middleweight Championship in a memorable encounter with Champion, Michael Bisping on November 4, 2017 after four years away from the sport.
However, despite the classic fight that earned him the Championship, St. Pierre's reign ranks as by far the shortest in UFC history.
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St. Pierre didn't lose the belt in the ring however, he vacated it due to injury. Suffering from ulcerative colitis, St. Pierre gave up the title, not wanting to hold up the Middleweight division.
It is surprising that one of the greats of the UFC should be front and centre of such an ignominious list but St. Pierre's nobility in giving up the title as soon after he was diagnosed is a testament to a man who puts his company before himself.