Probably Tetris. I just never get sick of it.
But if we're talking actual stuff you eat or ingest, then milk. I can easily go through a gallon of it in two days, once I had half a gallon with breakfast.
Carbonated beverages (preferably with a good dose of caffeine in it), assembling and painting Warmachine / Hordes models (even though I don't tend to play the actual game very often...), getting mad on the internet, and Disgaea 4.
My addictions would have to be coffee, Vincent D'Onofrio movies, Law & Order: Criminal intent, the music of Voltaire, cartoons and Deadly premonition. I need to have/watch/listen to at least one of those things a week or I am cranky as hell.