Answer:
3,333,360
this is by getting every dot, eating every fruit, and getting all four ghosts with every power pellet for the first 255 levels (after that, the game becomes unplayable).
While this might be the highest score, the best score is arguably a zero! This guy managed to get a perfect zero score in Pacman by dying three times without eating any pellets: http://www.youtube.com/watch?v=EuLj6Vblg_A. The trick is to move left and right at a rapid speed.