Druid. Going all tree-hugging nature-loving hippy and at the same time espousing trendy, incoherent "balance" claptrap would send me over the edge. My hat of the druid know no limit. Even being the cheesiest, most overpowered class in the game doesn't induce me to play one.
Pretty much any other core class, however, I'll play. Paladin? You betcha. Monk? Not a big fan, but the 3.5 version multiclasses well. Fighter? Love the feats baby. Barbarian? Me rage, me smash. Given a non-stupid oaf model (like the Arcanis val-Tensen hook), I can really get into it. Bard? subtle manipulation isn't something I can do forever, but I can be pretty good at it given the right tools. And the bard has those tools. Cleric? What's your flavor? Battle cleric, support cleric? I'll play them all. Rogue? I tend not to like roguey settings, but as long as we're not stuck fighting undead all the time, I can get into it. Ranger? I love the flavor; mechanically, I don't quite have it figured out, but I like the class. Sorcerer? As long as I get to start after level 4, I'm good to go. Suffering through the low levels is a pain though. Wizard? You betcha. (Fighter/wizard has always been my favorite combo). Even some of the expansion classes--Psion, Wilder, Psi Warrior, etc. seem like a lot of fun. I'm not so keen on things like Favored Soul, Samurai, Hexblade, Scout, Spellthief, and Warmage because they seem a lot more limited than the core classes, but I would be happy to play them if I needed a change of pace.
The only class I can't stand is the druid.