I refuse to vote on the poll, since it's so heavily dependant on the class. I play a lot of sorcerers and clerics, so I can't choose between wisdom or charisma as the main stat. Obviously, any spellcaster with a 13 in his main stat could never cast a spell above 3rd level. That would make them a pretty awful spellcaster, well at least by the time they get to 7th level.
Plus, if I made a thief, I'd want Dex. I don't see Con or Str totally necessary for fighters or barbarians, because I could make a salvagable fighter with a 13 strength, or a salvagable barbarian with 13 con. A thief with a 13 dex would be viable as well, but a high int would be really nice if that were the case.
Still, it boils down to a tossup between the three spellcasting stats - Chr, Wis, and Int.