I think Champions Online, because I genuinely thought they couldn't screw it up.
I learnt years and years and years ago that almost everything in life is better if you don't build it up so I don't read reviews, previews or game news, its not hard to work out what games will be good/bad without reading any specifics.
Spore was truly crap, but from the day they started really talking it up and doing presentations infront of crowds years before release of how awesome your animals would be, I assumed it would be complete crap because EVERY SINGLE GAME thats ever tried to do way more than every game before it, has failed miserably.
So spore wasn't a let down because I expected a game that reached so far it would end up massively scaled back, in an unplanned way and be crap.
Champions online though, how, how how? City Of Heroes is already pretty awesome but very dated and quite small for a MMO these days. I thought, yes, City of Heroes 2(maybe incorrectly), but it seemed obvious they knew what these guys wanted, same thing, new story, new character options, updated graphics engine, bigger and better. But they just took a good game idea and a great predecessor, and did literally everything wrong in the entire game.
Star Trek Online from the second I got in the open beta for Champions online, I knew was going to be crap so didn't get let down their either.
Other than that, Soldier of Fortune 3, biggest pile of crap ever, though again it wasn't surprising, SOF2 was one of the best MP games around for years, and decent/fun SP aswell. As soon as it got moved to the budget studio with no money and no manpower it was always going to suck hard.