If you must, Charisma. I'd recommend a rewrite of how action points work, though.
In a high-op game, though, probably Wisdom. Charisma is already the most powerful stat once you venture into crazytown, followed very closely by Int for its association with the game's most powerful classes. However, for your average op game, Charisma doesn't do much but boost skills that nobody's using to break the game in said environment.
