Well it kind of depends. Serra was a massive underdog and no one saw any scenario where he won. That being said his win was more "flukey" or whatever and GSP justified that original betting line in the second matchup. Dillashaw was a big underdog, I thought he had a better chance than people were generally giving him, but I still thought Barao would take it and I didn't think there was a way in hell TJ would outstrike him, let alone drop him on the feet and TKO him.
So if you're just looking at the matchup alone I think Serra-GSP was a bigger upset, but if you look at the fight itself I think this was a bigger upset cause he beat him in the way I thought he had no chance of beating Barao, and he dominated him 4.5 rounds before the finish.
A lot of people say Silva vs Weidman but I don't agree with that. Weidman was undefeated, and while I picked Silva (it's hard to pick against a long time champ) something felt different about Weidman going into it. The method of victory was more shocking to me than the victory itself. A 50-45 to 48-47 UD with wrestling would not have surprised me as much in that fight, mainly because of Silva vs Sonnen 1.