I'd have to say anything in the Tecmo's Deception series. And its only bad in the sense that the control layout, voice acting, and story are as bad as any in all of video games. And yet the gameplay is chef's kiss perfection. The gameplay is so good I hesitate to even call the game bad... but then you remember everything else about it is. Well; the sound track is bad but overall sound design isn't terrible, there's even a great cartoon bonk sound effect of the basin hitting someone's head and comedy squeak of someone slipping on the banana peel.
A really fun bad game is also the N64's Hybrid Heaven. Terrible turn based RPG, but the combat is full of professional wrestling moves. You move around in the game and when an encounter is triggered you are teleported early Final Fantasy style into a combat scene where you take turns dropping suplexes and DDT's on a variety of enemies. Leveling up lets you increase your pro-wrestling moves list... its hilarious. The joke wears thin long before the game is over, but the story is so bad there isn't a reason to even play it for anything other than the novelty anyway.
