The one Game.

the vampire masquerade: bloodlines.
damn i loved it so much, no vampire game ever was this good in my opinion -_-

So many games I haven't played from the comments :D
I don't know what to pick TBH, I'll either throw in half a dozen titles minimum or nothing. Favourite game is too mood-dependant for me :)

Bad Rats.

Warcraft :)

The Elder Scrolls here too

+1 for Morrowind (500+ hours on Steam)
Do like the Witcher series though.

Fallout New vegas

Bully: Scholarship Edition is my favorite game ever. ^_^

All Doom Series

Gothic 1,2 (especially Gothic 2),3 (with community patch).

Heretic although it's complicated,the second one was Hexen then Hexen 2 and then Heretic 2 but 3d person...Do i hear DOOOOOOOOOOMMMMMMMMM?Love!

Witcher 3, nothing even comes close.

Legacy of Kain: Soul Reaver (and every other game in the series): Just listen to the epicness,

https://www.youtube.com/watch?v=w2RMWWBXYbs

Borderlands

Commandos

I honestly don't have any. I jump from game to game constantly, so I rarely have a favorite for more than around a year. Below are a handful of games which are memorable, but not really "the one game".

Just Cause 2 comes to mind, but to be fair I was just a really big fan of the game at one time and still love the game for what it is. I don't play it anymore, nor do I think that the game is something for everyone - it's mindless open-world action, and not everyone can appreciate this kind of thing.

Then there's TERA which I suggest a lot of people to play, but the endgame is kind of shitty so I can't recommend it.

Then lastly, 100% Orange Juice. I don't play it much, but it's crazy fun with friends and is super easy to learn and play - I convinced 4 people I know to buy it when it was ~$1, because everyone should experience hating their friends over a cute anime board game.

Either Recettear or Long Live the Queen >.>

Crusader Kings II, only game you will ever need.

I'd love if the Space Quest port actually worked. And of course, get rid of that hideous point-and-click version for the first one and bring back the original.

Valkyria Chronicles

My absolute favourite? I don't think I can limit that to just one game or series. XD

I guess I'll just make a list (in no particular order):
Giana Sisters: Twisted Dreams;
Most of the Hyperdimension Neptunia series (Re;Birth2 being the exception), especially Re;Birth3;
Borderlands 2;
Darkest Dungeon;
Recettear: An Item Shop's Tale;
Orcs Must Die! series;
Black & White;
Half-Life series;
...
Devil May Cry series; Serious Sam series; Painkiller: Black Edition; Bioshock series...

AND A LOT MORE! DON'T MAKE ME PICK JUST ONE! I LIKE THEM ALL! O_o

Well, not one, but I'd say the Mass Effect series as a whole.