I'd definitely have to say [Prototype] and Street Fighter IV. I usually don't expect things going into games so I won't disappoint myself or anything, but Prototype... That was just horrible :I that is literally the only game I forced myself to play, got halfway through, sat back and said "Why did I buy this..." Got Street Fighter because I kind of enjoyed the arcade cabinets I would play at Pizza places every now and again back in the 90's and decided to give it a shot. Wasn't bad, but just wasn't my kind of game really. Probably would be better using a Arcade stick setup but I don't really wanna opt the cash for something I'll really only use for one game. I don't really regret owning Street Fighter so much as I regret paying the full $60 for it.
On a side note, I also regret picking up The Elder Scrolls IV: Oblivion. I liked Fallout 3, so I went and picked up the Oblivion GOTY edition for $20 over Dead Space (Which I ended up picking up later anyway for the same price and absolutely loved it). I got out of the sewer and sat there saying "WTF?", something I didn't stop saying even after completing the firghters guild and getting 3/4 through the storyline. Game just had me lost from the get go. It was only $20, so I don't feel as bad as I do with Prototype of course, but I don't like the 250 or so gamerscore for it sitting on my gamertag.
I apologize if any of these offend anyone, they just didn't suit me... Except Prototype, I'm not really sorry for that game... That game was horrible. I liked it better when it was called Spiderman 2.
Edit:
Oh! Fable III! I definitely regret buying that more than Street Fighter and Oblivion, but Prototype is still worse. What kind of game claims to have so much innovation and then end up being exactly the same as the second game, both graphics and location wise, and makes making money a key feature to the game... What is this an RPG or an introduction to capitalism? The story took me 7 hours to get to being king, worked as a blacksmith for 4 hours so I could get the good ending and save everyone, and only had 60k of my needed 8.5 million (you go 2 million in debt choosing the good options) so I cheated and the game ended 15 mins after I got all the money... Seriously what game is more working as a blacksmih, buying and managung property, and collecting flowers, books, gnomes and keys than actually being some kind of RPG when its advertised as an Action RPG? /hate rant