I'd say Hearts Of Iron (preferably Hoi2 and the Doomsday addon). There are too many ways to do it to go through them all. Basically, the outcome of the game is never the same. Unless you play as the same nation over and over again. There are so many small events that can change the course of the game.
a melting spool of beggar negative frames
random maps, random drops, free multiplayer on servers (if you own a legit key) I still play it after 6 years, even though not that much these days... But yeah, Diablo II has the highest replay value for me.
