There are really only two games that I have ever gone back to to replay. It's really just not something I do, so a complete rarity to replay a game, for me.
For singleplayer it would be Prototype. I have played through the full story 3 times and, as I said, any replaying for me is strange so 3 seems a lot to me. I don't know what it is about that game, but it's just fun to play, so maybe that's just been enough on its own.
For multiplayer it would be Civilisation 2. That is the only game I've reinstalled to go back to for multiplayer reasons and it is only to play with family. My sister and my brother still get a kick out of playing that game together, so every now and then, when we meet out, we tend to drill out a few sessions in game.
Honourable mention: I almost replayed Mirror's Edge once, but I wasn't really interested on playing through the same content. I guess I wanted another experience like that, but don't really know where to turn. Really, I'd just like a Mirror's Edge 2, which I believe they are supposedly working on, so if that's true, hopefully that will be good.