Diablo 3. Error 37, monetized player transactions, boring itemization, authenticators, Jay Wilson's "fuck that loser" slip up, the hilarious writing, etc. Diablo 3's strengths (and it does have its strengths, combat being one of them) are overshadowed by all the bad decisions Blizzard made. The game did sell extremely well (10m I believe), though Blizzard didn't deserve a fraction of those sales, as bitter and crabby as that might make me sound.
But every cloud has a silver lining. D3's entry into the market actually sparked interest in other arpgs - pre-orders for Torchlight 2 increased by 40% when D3 launched. Path of Exile is enjoying similar success, and I'm sure part of that is from people looking for a worthy D2 successor.
So, D3 was disappointing, yes. But just it's existence has benefitted smaller studios, which is great. How many of us remember saying "those games look good, but when D3 comes out they're dead". Pretty sure I did...