I don't think I play a lot of bad games in general, but the worst games I've ever played were probably Facebook games -- like I've seen stuff like Allies and Empires get lauded as the best that FB offers and yet, when I played it I just kept thinking about much better simulation games I've played for Windows and on consoles. The worst FB game I remember playing was called Dictator Wars. It was obviously in the vein of Mafia Wars, but in this one it was specifically impossible unless you spammed the hell out of your friends. The game got shut down before I even had a chance to uninstall it.
Atari Pac Man. So horrible, it still gives me nightmares. Graphics = atrocious, playability = even worse. I regretted the purchase from the first second I turned on the game. I just threw up in my mouth a little thinking about it... gotta go........