Some of you are making me feel so old. Are you just really young, or have you actually not played a bad game until 5 years ago or less? XD I have a few games that come to mind as the worst ones I ever bought, but only one or two are recent ones. And they aren't so much 'bad' games as they're bad for the value. Force Unleashed 2 has decent gameplay and a decent story, but it's very very short with zero replay value. Fallout New Vegas isn't bad (once you get past the bugs which quite a few have been patched away), but with so many reused textures, environments, near-identical gameplay, it feels like an expansion pack for Fallout 3 that drags a little too long (like this sentence XD).
Okay, the real worst games I've ever bought in Top 3 List form:
1. Superman (N64) - this game is legendary, so I'm not sure I have to explain; but... Horrible graphics, a lifeless environment, and padded out by an insane obstacle course that was impossible to navigate with the broken flight controls.
2. Justice League Task Force (SNES) - I played this in one sitting, not even trying all of the characters, then never touched it again. A very boring cookie cutter fighting game that pales in comparison to other fighting games. When your fighting game could easily be recreated (and improved) in Mugen, something is wrong. I had more fun with Shaq-Fu.
3. Alone in the Dark (Xbox360) - I know, I'm naming a more recent game, but this one stings a lot. This game looked so good in previews, especially the item creation and fire effects. It could have been a great game if not for some horrendous design choices and tons of bugs. And there's a button to control your eyelids (blinking and closing them) - not sure if that's a positive or a negative...