Civilization IV. I have a lot of RPGs that I spend a lot of time on, but none of them came close to the Civ IV. I failed a class playing this game. I would've ruined my marriage if my wife wasn't so addicted to the game as well.
Although realistically I'd also have to include Dark Age of Camelot, as I spent a good 4 years playing that game. In the end I think I was over 1000 hours played on my main. That's more than 41 days of my life playing DAoC.
Another good one would be Lufia 2: Rise of the Sinistrals. It's a pretty standard entry into the SNES RPG library, except for one thing: The Ancient Cave. It's a randomly generated dungeon with 99 levels of increasing difficulty, where you start at the beginning at lvl 1 with almost no equipment and build your way up throughout the dungeon, only taking with you certain treasures when you leave. Brilliant, brilliant game and sucked many a day from my life.