Good question. Quite a few this gen, but these are my biggest regrets:
PS3 - BLAZBLUE: I can see why people say it's good, but it made me realise that I'm not as good at fighting games as I thought I was. Really steep learning curve and I can't be bothered really.
PS3 - KILLZONE 2: Picked this up for £20 and not sure why. Decent game, but I was never gonna play this as much as I play COD and I can't be arsed with the trophies.
WII - LITTLE KING'S STORY: Again, a decent game but I knew deep down when I paid the £10 that I would never find the time to play it. I gotta stop impulse buying.
DS - MEGA MAN ADVENT: Guess I just wanted to have another Mega Man experience like X4 and X6 but on a handheld and two generations after those games. Man I was wrong.