For sheer slack-jawed awfulness, probably Lula 3D. It's meant to be an erotic adventure game (although if you find it sexy, you have problems) about a porn diva in search of some missing actresses, which just goes from bad to worse to indescribable.

It's a game so sloppily made, there's a conversation where they forgot the English dialogue, leading to one of the baddies talking in German and then randomly adding "Huh? Avacados?" One of the puzzles involves firing a pet dog at Mt. Rushmore. The entire world has random naked people having sex just scattered around it for no reason. There's a sequence where the main character bumps into a thief, hitches a ride, and the two of them promptly get in a gunfight with the cops. Half-way through, the developers actually have the guts to pretend there's a plot, which would be cute if it wasn't so unbearably tragic. Oh, god, it's so terrible.

The highlight is near the end though, when Lula is captured by the villains and the game suddenly changes into a shooter. For no apparent reason, they just drop her off in a drugged state in New Orleans, and you spend the last few screens fighting skeletons and zombies in a psychedelic haze, all with horrifically bad controls. When everyone's dead, Lula's only real response is basically "Well, that's that. Time to go home!" And then she has a party.