Demon's Souls, Dark Souls, Dark Souls 2 - The Souls series is extremely replayable. Not only have I rolled multiple characters in each game (strength based warrior, high dex no shield, mage, cleric, and combination of) due to the extreme range of play possible, but I have also beaten the game multiple times on the same character getting to NG+, NG++, NG+++, etc.
Diablo 2 - There are so many different ways to play, new loot to find, and it is such a easy game to get into.
Morrowind - So much content in this game. After playing 400 hours via multiple characters throughout the last 10 years, I still haven't seen it all.