It's gotta be Final Fantasy XIV.

I've spent weeks just fishing.

Deus Ex with mods.

its a tie up b/w csgo and skyrim...

dont judge me

Only one game?? What a nightmare. I would want something big with a lot to do. I’m inclined to pick Morrowind with a few mods.

For the rest of my life...

Can't think of any games I would want to play for that long (hopefully).

But I love wandering the hills, valleys, and deserts in Read Dead Redemption -- watching the sun rise and fall. If wandering on horseback is considered playing, that would probably be the game I'd enjoy most over the remainder of my days.

Star Wars Roleplay (D6, not D20 or FFG).
Computer games get stale, and I already played those of infinite replayability to death, and while I'll play them some more yet I wouldn't be able to stick to one for the rest of my life. On the other hand roleplay never gets old so long as your group is even slightly decent, it falls apart when said group breaks up because someone kicked the bucket, had to move far away, got drowned in work and so on...
Other roleplaying games like Ars Magica, Dark Heresy or Fantasy Roleplay, Legend of the Five Rings, Shadowrun, World of Darkness or D&D are fine as well, I'm not that picky, but offhand Star Wars came first to mind.

True (never played RDR though). I just think that people who likes adventuring in general would opt for open world games to play for the rest of their lives simply to enjoy the great sceneries that such games provide, probably due to the atmosphere as well. For example, I chose to go with S.T.A.L.K.E.R. Anomaly since I've always enjoyed the atmosphere that the original games have (the one I mentioned is just a standalone mod). I'd probably spend most of my time being afk in the game while enjoying some campfire time with the NPCs.

Can I cheat and choose a compilation or demo disk?

Neverwinter Nights. If I'm only going to play one game for the rest of my life, it's going to be a game that I've already sunk hundreds of hours into without even having played every possible module available.

the Binding of Isaac: Afterbirth+

I'll mention a list of games that I replay over and over and still enjoy. If you want an answer of just one game, it's the first one on the list.

Broforce
https://www.gog.com/game/broforce
I'm often in the mood to zone out and play a game without thinking and this is the game I play most when I'm in that mood.

Halo Combat Evolved
It's not on GOG. I have the original disc version with a more recent update and a community patch which adds console commands for adjusting things like animation interpolation and the FOV.

Alan Wake
https://www.gog.com/game/alan_wake
It's just easy for me to pick up and play over and over.

Red Faction Armageddon
https://www.gog.com/game/red_faction_armageddon
I like mindless linear games.

You know, if only the game had some sort of a skirmish mode where you could fight against AI whose difficulty could be adjusted (just like Star Wars Battlefront 2 (2005)). It'd be fun battling against other AI spartans in Blood Gulch. That'll add a lot of replay value to the game. But then again, I doubt the AI could replace the kind of absurdness that only real players could pull off when playing the multiplayer mode of Halo, you know the random stuff.

Probably Morrowind. It's in my top 5 and is pretty packed with content and variety.