The title pretty much explains it all.  If you were only able to play one game for the rest of your life, one, single, game, what game would it be, and do you think you'd be happy with it?  I'm really not sure what I would pick, this is a tough question, lol.  I'd probably have to go with a long epic like a Final Fantasy or Xenosaga 3 but maybe I would go with something I've always enjoyed replaying, like Eternal Ring (beat it three times and on my fourth replay the disc is now glitched)

I would be happy with Tetris DS because there are literally an infinite number of games you can play on it.

BTowns : hmm,can you buy game add ons?or install patches/play seasonal matches?
Id probably get bored no matter what i chose so id probably go with a game like half life 1 or 2 (on pc)because you can play user made levels,so it'd not end for a long time.
If you were allowed add ons that would mean i could also use garrys mod which is great for making new items.
If you dont include updates/online/add ons id go for one of my 2 favourites. Bioshock 1 or hitman bloodmoney,given iv completed each about 5 times(honestly,i just love them)
Iv literaly never played or seen gameplay of any of the games you said so il have to look them up.
I think if this were the case id play the game (to completion)about every 2nd month so i didnt end up hating it.

My first thought was a game like Skate 3 because I love the series and you can make your own park and move items in the world around to make an infinite number of different possibilities for epic parks and moves, but it seems like I would get bored of just skateboarding after a while.

My next choice was gonna be Golden Sun: Dark Dawn because I love the game and still haven't taken the time to sit down and complete it.  If it was my only game I'd have no choice but to complete it, and play through it multiple times (to make sure I unlock and beat everything).  But doing the same quests and story multiple times may get repetitive (especially since I don't know exactly how long the game lasts).

Also either a Final Fantasy game (FF7 probably) or Grand Theft Auto 4.  Final Fantasy 7 lasts a while so you can enjoy playing through it for extended periods of time, but in Grand Theft Auto 4, even if you get bored of the story, there's always havoc to wreak in the game.

If I had to choose one game though, I would probably say Golden Sun: Dark Dawn.