Final Fantasy XII. I can still remember the plot and characters from 10, seven years on. But a few months after playing a random game full of one-dimensional characters, random fights and a completley sterotypical and forgettable plot, I can only remember Basch. And only because he's a poor man's Auron.
Also I regret ever buying any of the Pro Evolution Soccer games, as I have lost days, literally, playing that game.