This was a tough poll. Soo many bad classes. Most of them, though, only for their design problems.
I voted Psionicist, merely because I think it makes more sense as a kind of wizardry in a D&D world. The idea that it was a
totally different type of thing than magic always rubbed me the wrong way. Its also a class the seems to attract "problem players", IME.
That's not to say the Psionicist didn't have competition:
Monk - is he a fighter? a rogue? a totally different animal? Whatever he is, he's been kind of an oddball in DnD just because
Kung Fu (awesome show) was on TV back in the day. That might be okay, but the monk almost always seems to be "held back" by a slow progression of abilities.
Sorcerer - much like the psionicist...I'm a wizard, but different! But not different! At least the sorcerer isn't trying to use the word "molecular" in a world where the 4 classical elements actually turn out to be "things."
Fighter - whose only problem is that he is overbroad, non-distinct, and/or underpowered in almost all versions of this game. Don't believe me? Try playing in a BECM campaign with three Fighter PCs.
Wizard - What?! Yes, really. Wizard was actually my second choice. Its always been a train-wreck of a way-too-broad and easy-to-get-out-of-hand class. It seems like its really hard to get a class right when its only description is "I do magic." Partially a victim of the endlessly increasing list of ever mightier magic, this class needs to be fixed somehow. Just think of the number of threads you've pointlessly argued in because of this class.
Of course, all these problems vary in severity from edition to edition.