It used to be Constitution back in 2nd edition, where having stats that gave you a bonus (or negative) wasn't as likely as it is now.
The general sort of character I prefer to play could likely best be described as a swashbuckling sort; not exactly, as I rather like cleric's, but even then, they tend towards following gods of a more roguish intent. Which means I prefer them being rather nimble (Dexterity), savvy, clever and witty (Wisdom and Intelligence), who can express that rather sharply and confidently (Charisma) and who, while perhaps not the strongest, is still muscular (thus, Strength still has importance).
However, in light of the shift in how abilities work in 3rd now, having played a character with low Con, I feel it's a bit more integral to have now. Not that it wasn't before, just not quite so much.
As such, I really don't have one dump stat, per se. I guess I'd say it's Wisdom, but a decent chunk of my character concepts are cleric-types. When they're not, though, I think that's often enough my dump stat. With priestly folk, though, it probably shifts between Dexterity or Strength, depending on just what I'm going for. Whle I often don't prioritize Charisma, even when it's non-essential, I still prefer it to have a positive modifier, if only a +1.