Left 4 Dead. Each campaign, dozens and dozens of times.
There aren't many games where I like doing the same thing over and over. If it's an RPG, I'll probably have one or two save files with 100+ hours but only a few playthroughs to my name.
Oddly, I hold the high score world record for Rampage: World Tour on the N64, but I've only beaten the whole thing twice. Once, just to see the last level, and the second time to set the world record.