Oh boy, there's several contenders.
Marvel: Ultimate Alliance- The GBA version, that is. While the console version is a Diablo-esque action RPG, the GBA version tries to be an early 90's beat-em-up. And it fails. The gameplay is broken, the graphics literally look they were made with MS Paint, and even had it been good it only lasts an hour anyways. It's just a pointless waste of time. On a side note, the DS version of MUA2 is also horrible thanks to non-existent AI and the worst camera I have ever seen. But it's not quite as bad as its GBA predecessor.
Star Wars prequel games- Or to be more specific, Phantom Menace on PS1 and Attack of the Clones on GBA. I feel kind of bad knocking them since I enjoyed them as a kid, but looking back...good god, what was I smoking? I give some props to Phantom Menace for using the dialogue system that would later be used in KoTOR, but that's it. Episode 2 is a terrible bland beat-em-up with really bad controls.
The GBA/DS version of Episode 3 was pretty good, though.
Sonic 2006- Must.Not.Write.Essay. To sum it up: it sucks. It really, really sucks. I hate everything about it, from the lack of speed, the alternate gameplay styles, the controls, and the insultingly stupid story. And this is coming from someone who liked Shadow the Hedgehog, so believe me, it's bad.