Which game will you most likely come back to occasionally for the rest of your life? I’d like to limit it to one game, otherwise it’s too tempting to just list all your favorite games, which isn’t what I’m looking for here.
It’s hard to leave out my few sacred cows, but I suppose If I were only able to play one game for the rest of my life it would be Tetris.
Master of Orion II.
Age of Empires II.
Yep, Tetris. The only game I’ve ever played in my sleep.
For certain values of video game, my choice is the Total Extreme Wrestling series. It’s a text based fantasy booking simulator where the player assumes control of a wrestling company, hiring/firing talent, booking shows, etc. There’s a new version (TEW2013) due out this fall.
X-Com:Enemy Unknown
Weirdest thing is ? I’ve never actually finished it. Always get either bored first (it can be pretty samey/repetitive) or massively screwed by the random unfair crap the game throws at you.
And yet it’s still one of the best games ever. Funny how that works.
Tetris DS
Super Mario Galaxy 1 & 2
Yoshi Touch & Go
…basically, my SNES. 
The Civilization games. Oh, and Prototype 1&2, just the smashy part is fun for stress.
Colonization
Civ2
Seriously guys.
The most recent Civilization (I’ve enjoyed all its iterations and expect that trend to continue) is probably the game that I return to the most.
Super Mario World
I still come back to Kid Icarus from time to time–it was one of the first NES games I owned. And when I do, I usually run through it several times to get the best ending. 
Apparently, Fallout: New Vegas.
I have Skyrim, Red Dead Redemption, Assassin’s Creed: Brotherhood, Bioshock 2, FIFA 12 and Dead Space all sitting there unfinished because I can’t tear myself away from F:NV for very long. I’ve promised the wife that, after I finish this New Vegas run-through, I’ll play a different game for a while.
I hope you didn’t get bored of that game, considering it’s not been released yet (Oct 9, 2012)!
The 1994 game is “X-COM: UFO Defense” (NA) or “UFO: Enemy Unknown” (EU/Elsewhere).
Unless you mean “X-COM: Enemy Unknown” on PSX, but I’m not sure we’re supposed to acknowledge that one, certainly isn’t the “real” title.
I have a ridiculous amount of time into Skyrim now (and Morrowind et al., but those didn’t measure time), although I might get bored eventually. I don’t know if I can play any game forever, but high modability helps.
Skyrim and Fallout games really don’t do it for me. Don’t get me wrong, I’ve sunk many hours into both of them, but it ended up feeling like a chore after awhile. At some point I’ll return to Skyrim, but I’ll probably just bust on out with the main storyline until it’s over.
The one game I reinstall every few years and play through, though, is Starcraft. I can’t get enough of that game, it seems. The sequel not so much, but the original (and Brood Wars)? Awesometastic gaming.
Civ 4 and Ocarina of Time.
Marvel vs Capcom 2.
If I haven’t gotten tired of it by now, I’m pretty sure I never will. It’s still incredibly fun whenever I have anyone to play.
And there will never be any game like it using sprites, so it will never be replaced. 3d models of Marvel characters don’t have near the impact of cartoon-like animations.
Err, yes, I meant the one that came before Terror From The Deep :o.
Speaking of which: fuck *that *game. Fuck tanks and weapons that won’t work overland. Fuck cargo ship terror missions. Fuck aliens camping inside closets forever. Fuck lobstermen. Fuck unwinnable saves because one mandatory captured alien just won’t spawn any more. Fuck lobstermen some more.
Basically fuck everything about TFTD from orbit. It’s the only way to be sure.