My initial thought was Stannard winning Het Volk, because that was the most 'make me laugh out loud in disbelief' moment. But thinking more, I realized that was more disbelief at QuickStep losing than Stannard winning... I always had the feeling he could be good since he wasn't working, but I figured they just had to break him with one of their attacks (or 'attacks' in quotes if it's Vandenbergh).
Sagan in Cali was pretty wild, although I'd say Alaphilippe winning on Baldy (and winning the overall but for time bonuses) was even more crazy, considering he'd never, ever shown anything on more than a hill before. Sagan, at least, you could see from breaks he's been in that he's okay with mountains, just not upper echelon. Alaphilippe's 2nd at Liege might be the surprise result of the year (or Benoot at Flanders), but wasn't a win. Seriously, a 2nd-year pro finishing 2nd at Liege? Totally nuts.
I gotta go with Dumoulin in the Vuelta, though, on the stage he beat Froome and Rodriguez. I feel like there aren't too many other riders in the peloton that when they go past you, it's inevitable that they're not coming back. But there it was, Dumoulin was already up the road, Froome and Rodriguez broke free of the pack, Rodriguez waited until they caught Dumoulin, waited... then went! And that was it... but it wasn't it. They came back. Even Purito afterwards said he didn't have the legs but he had hoped they would think that he was gone and just give up. Then Froome goes past and is in the clear, another guy that never comes back once he's gone. Then Dumoulin, who had already been up the road, comes back and blows by him! That would be like if Cancellara jumped from the peloton with 500m to go, got a gap, got passed in the bunch sprint by Kittel and Griepel, and came back to overtake them. Watching that was. just. puzzling. Like, what the hell did I just see? It's like when Landa won in Burgos for Euskaltel in 2011 or 2012 or whatever that was. Just really impressive and crazy.