I'm guessing that for Anakin to finally go to the dark side in Episode III he'll have to kill a major character. Who do you think it will be?
Options are:
Mace Windu - the most powerful Jedi after Yoda, I believe (he helps run the Council, anyway). Big props for the guy who can kill him.
Padme - yep, Anakin kills the good lady wife, obviously after she gives birth to that near-incestuous brother-sister team of Luke & Leia. And keep in mind that Darth Vader wasn't aware that Luke had a sister until he read Luke's mind in "Return of the Jedi", so watch for another plot hole in Ep III if we see Annie with the two kids.
Jar Jar - he kills Padme's second-in-command, and nobody thinks any worse of him.
Count Dooku - that's right, the major character doesn't have to be a good guy. He obviously doesn't appear in IV-VI, but then again he might end up in a retirement home waiting for reruns of "Matlock".
Any other suggestions? Then again, maybe he doesn't kill a major character, and just turns to the dark side after shoplifting and the occasional bit of vandalism.