Despite sounding cliche, I think Ultimate Warrior defeating Hulk Hogan clean at WM6 was one of the biggest widely viewed upsets of all time. Despite the fact that the buildup to that match revolved much around the "unstoppable force meeting the immovable object", it's important to truly understand the context of that bout in the grand scheme of things:
1. Though fading ever so slightly, Hulk Hogan was still in his prime in early 1990. He still had that unbeatable aura around him. He was still turning back every challenge put in front of him. Wrestling fans still believed he was unbeatable... invincible even.
2. And as far as WWF promotion as a whole goes, he was unbeatable. Hogan rarely ever lost and when he did, it was by something innocuous like a count-out or double-DQ. Even in losing (if you can even call it that), Hogan typically looked (or came out looking) stronger than ever. He'd eventually get his proverbial revenge with a big boot, leg drop, and post-match posedown culminating the festivities. For many, many years nobody broke this formula. Hogan, quite simply, didn't ever really lose, especially at a high profile event like Wrestlemania.
3. As for the Ultimate Warrior, he was still relatively new on the scene. Yes, he had a huge following by the time of WM6 and its debatable that the "little Warriors" competitively numbered the "Hulkamaniacs" in a side-by-side comparison. Nevertheless, Hogan had an established track record of beating every single challenge that had been put in front of him, most times in overly convincing fashion. By comparison, Warrior had beaten guys like Hercules, Rude, Haku, and Andre... all guys who Hogan demolished years prior.
I was a huge Warrior fan at the time, and I didn't even think that he was going to win at WM6. The best I had personally hoped for was some kind of a draw, leading to a continued fued that might have culminated at the next Wrestlemania. Based on the promos leading up to the event, I actually thought Warrior was going to turn into a "bad guy" so that "unbeatable Hogan" would have a new adversary to annihilate in the end.
But that didn't happen. Warrior not only won the match, but he pinned the biggest name in the history of wrestling in his prime, who had been booked for years as "unbeatable", clean, for the title, and on the biggest show of the year. I don't care who you are or how you're built up in the months leading to an event. Doing what essentially equates to the "impossible" automatically propels you into "greatest upset of all time" status. That is precisely what the Hogan/Warrior bout at WM6 represents to me and quite frankly, I can't think of anything off-hand that would even come close.