Clerics. I practically have to beg to get a divine spellcaster into the party, but they always seem to end up as favored souls, paladins or rangers, the last two of which don't even count.
Quite a headache for me to balance play when nobody in the party can effectively disarm traps. Believability went down the crapper when I gave up and started putting traps in anyway, only to have the Monk or Paladin deliberately walk into them (and otherwise take the lead in the marching order to “trigger” unseen traps) simply because their saves and abilities allowed them to shrug off most trap effects anyway.
I'm shocked by many of the people here answering rogue. That is one of the most common characters played in our game. It's such a universial participant with moderate combat abilities and great skill set. Who can turn down all those damn skill points?