Games That Keep You Coming Back?

The Guardian Gamesblog kicked off a great discussion on their site, talking about games that just keep pulling you back in. Games that, even if you've played through them once, you just have to pick them up again. eToyChest and Kotaku both have related threads. So, what about you? What are some videogames that, even years later, you just have to play through one more time? For me, besides my ongoing fascination with World of Warcraft, Star Wars Galaxies, and Everquest II, there aren't that many that needed more than one playthrough. Both Half-Life titles, of course. I needed to play HL2 just to get everything I missed the first time. Jedi Academy and System Shock 2 required additional plays to try the game at a different angle. Similarly, I've played through the Diablo titles more than once each, as there's just so much clicking to be had. I somehow managed to avoid the gravity well of Civ4 for the most part, but Civilization 3 was almost the only game I played in college. Good times.

Comments

System Shock 2!

[PontifexPrimus]

... and Deus Ex (played that one on my last four computers). Both great games with extreme replayability, since they offer so much customzing options for your character. And, what's more important, those choices actually matter in terms of gameplay and are not purely cosmetic, resulting in sometimes radically different games.
Don't get me wrong, I like (for instance) the Splinter Cell series, but if you don't sneak in the levels where you're supposed to sneak, you simply lose the game; when I've made it through such an obstacle course once I see no motivation to do the exact same thing again the next time through.