This depends very heavily on what character I'm playing.
If I'm a fighter, I choose charisma as my dump stat. If I'm a cleric, I choose either dexterity or intelligence as a dump stat, etc.
I chose Charisma in the poll, however. Every single ability score except charisma can provide benefits to a character outside of skill checks and class abilities. For example:
Strength: Affects melee attack rolls, melee damage rolls, and how much weight a person can carry.
Dexterity: Affects AC, ranged attack rolls, and reflex saves.
Constitution: Affects amount of hit points as well as fortitude saves.
Intelligence: Affects how much skill points a character recieves.
Wisdom: Affects will saves.
Charisma: Affects nothing that benefits all characters. Only useful for certain skill checks and class abilites.