It doesn´t have to be the likes of Douglas- Tyson or so, you can pick your personal favourites. One of mine is Jersey Joe Walcott- Ezzard Charles, Jersey Joe fought 3 times yet for the world title and always lost (he was robbed against Louis, but that´s not the point now), he lost the 1st fight against Ezzard clearly, and than in their rematch with 38 years he dominated the champion and brutally knocked him out with a picture-perfect left hook...
For a more recent one, how about Hamed v. Barrera. Robinson v. Turpin I is hard to beat... And a personal favorite, which comes up a lot in this forum in various categories (!) Barkley v Duran.
I loved Hearns/Barkley I. Barkley's getting the crap kicked out of them and he changes that with one punch.
Brewster beating Wlad in there first fight, he took a ton of punishment just to land his shots in the middle rounds. Brewster proved to me if you hang in there and tough it out, good things can happen.
Barrera's win over Hamed wasn't a massive upset. The observers who went with Hamed were mainly picking him on power alone. Barrera had shown prior to the fight that he could stand off and box, but nobody expected those tactics against Hamed. The majority went with Hamed, but Barrera had many believers as well. Including myself, as I picked him to KO Hamed around the 6th round.
I think the Sanders fight was a much bigger upset. By the way, any idea what the odds were in his first fight with Brewster? I have a "10 to 1" figure in my head all the time, but i cannot believe Brewster to have been anything less than 2-1 or 3-1. Maybe Sanders was 10-1? That was a giant upset anyway, Wlad was the #1 contender for over two years straight and on a good winning streak. I would like to add both Golota vs Bowe fights. All his fanboys try to make it sound asif he was shot, but in reality, Bowe had just stopped Holyfield in closing the trilogy as winner and was seen as one the best heavy in the world together with Tyson and Lewis. Golota was barely in the top10 yet he gave Bowe the worst beating of his career, not once but twice. Although Golota did lose both fights, but i think it was a huge upset to see Golota just easily manhandle Bowe on every area.