Does Q count? Because that'd be awesome. Sadly, I don't count myself qualified to make twitter entries for an omnipotent being, but being Q, I do suspect he'd bypass the character limit.

For more classical stuff though, hrm. Probably Lucifer Morningstar, frankly. I very much loved the Carey/Gaiman take on him, and would imagine even short bits of thought would be interesting.

Also, Cobra Commander has a twitter account, which is usually terrible, but is amusing upon rare occasion, it seems.