I have 3 games I regret buying, but at varying levels of regret.
Mafia 2: I regret 100%. It was so short, and not too fun. I heard there was an open world and thought it would be like one of those sandbox type games where you can go around taking over shops and stuff, like my PS2 Godfather game. I was so horribly mistaken. Every time I look at my Steam library now, I can't help but be reminded of the mistake that sits there.
Black Ops: Wow I regret only 66% or so. I don't play it too much, and I have it for PC. If I had it for 360 or PS3 I'd play it more, but I don't have a console anymore. The single player campaign was...meh. It gave me a massive headache by the time I finished, and I really just plain ol' hated it. I expected by the name "Black Ops" that it would have stealthy parts mostly, where you do things like infiltrate a soviet base, gather some intel, and leave. But then again, that wouldnt be much on an FPS, so w/e. I don't play the multiplayer too much either, cause its laggy and I just suck too much with my computer. Not to mention the hackers. The only part I play that much of is zombies, and I only play kino der toten. five always has a bunch of guys who wanna do the table glitch, and DOA just doesnt transfer to PC well at all. I def. like kino. Overall, if they had a version that just had zombies, or maybe just single player or w/e that would be better.
Civilization V: I regret about 50%. Civ 5 was a sorta fun game, but I feel I could have had the same if not more fun playing Civ 4 or even a historically based RTS. The only reason I got it was cause I like those historical games, and it's games like that that got me into history, which is now my major. Also, I hate myself for buying some of that DLC. So worthless...