The worst game I own has to be Nanostray. I don't even know what the point is...someone got it for me as a gift and I've played it one time, never again. Cooking Mama just annoys me after a while.
I tend to look at reviews and trailers when buying games but there are the odd games that I don't bother with since they're part of a successful series. Like Advance Wars: Dual Strike. That game was horrible ;~;