There is an obvious answer to this question for me, and amazingly, no one in this topic has said it.
Maurice Smith.
Smith defeating Mark Coleman is by far the biggest upset in MMA history. Serra-GSP doesn't even come close.
Mark Coleman was seen as unbeatable. He was 6-0, had obliterated his only serious competition in Don Frye AND Dan Severn, and was this insanely powerful wrestler who would just bludgeon opponents with ground-and-pound, including his lethal headbutts. (Legal back then)
Meanwhile, Maurice Smith was a washed-up kickboxer with a LOSING record of 3-7, including a pathetic 1-6 in Pancrase. He had been dominated by much smaller, inferior grapplers than Coleman (Tamura, Ken Shamrock, Suzuki) and even a smaller, fellow striker in Bas Rutten. His best victory until then had been his last fight, a knockout of Kazunori Murakami, a mediocre grappler who would have been decimated by Coleman victims Frye and Severn, nevermind Coleman himself.
Keep in mind that a striker had NEVER beaten a half-decent grappler at this point in UFC history. And the idea that a guy as mediocre as Smith would defeat the unbeaten golden god Coleman?!
Utter insanity.
