That's a tough one. I don't think there really are five games that could alone satisfy me. I guess something like Tetris or Columns or Puyo Pop are timeless and infinite, but those kinds of games are only good for short plays. I wouldn't want to play them everyday for the rest of my life. Nor would I want to play any game or five games for the rest of my life. I don't really like multiplayer games so something like WoW or online shooters wouldn't be for me either.
I guess I'll go with these for the reasons listed below.
Portal 2
Fan made mods and levels could keep it entertaining for a long time. Every few years I can see myself replaying it for the story like I re-watch a good movie. It has the best of puzzle and action games. Puzzle games are timeless and good action games don't get tedious.
Halo 2
I've nearly played Halo CE to death even though it's still my favorite Halo game. Halo 2 has a more interesting story, more environments, and you get to play as the Arbitor which I've always loved. I almost put Halo 3 here but if I had to choose just one I had to go with Halo 2. If I could I'd put the whole Halo series here because in my opinion they go together all or nothing, but I can't live with no Halo so Halo 2 it is.
Panzer Dragoon Saga
It's the best JRPG ever made. I can see myself playing this every few years like I watch a good movie. My current favorite RPG is the Mass Effect series, but I can't choose just one and I'd hate to never play PDS again. If I could do series instead of single games then Mass Effect would win out.
Zelda Ocarina of TIme
It's still my favorite and I can see myself playing this once a year or at least every two years.
Forza 4
Racing games are great slow burn games. You can play them for years and years and never master all the tracks to perfections. Forza has the added bonus of letting you fully customize the cars so even if I got bored with the races I could paint cars forever.
