For me, it's Terraria.
And Plants vs Zombies :D

Dark Souls (1)

Gothic 2

Chromehounds! Thank fuck M.A.V comes out in 15 hours! Whoop whoop!

Quake 2 multiplayer.

Ocarina of Time singleplayer.

Doom 2

For me, Skyrim :)

Well, oldies included: DUNE 2! the one and only game (that said, its contender is Master of Magic...)
Mid gen games (~90's): Fallout 1+2 - i consider them one game, diff story, same playability.
current gen? honestly, i don't know. there are some good games, and some crapy games which are really fun.

I think I could never pick a favorite game of all time. There are so many differtent aspects to games, you can't really compare them all.

Storywise I'd say Deus Ex: Human Revolution. Storytellingwise I'd say Spec Ops: The Line. Gameplay probably Fallout 2. Graphics and style I'd say Skyrim. Versatility would be ArmA II. Best franchise probably GTA. Most time spent would be WoW.

But still there are other awesome games I really enjoyed but that don't really fit into such categories, like DayZ or Antichamber. Also timeless classics like Monkey Island or Day of the Tentacle...

Also with about 20 years of gaming on my side, perception of quality changes over time ^^

Dark Souls 1 (haven't played 2 yet) or the first 2 games of the banjo kazooie saga

Right Now it's PAYDAY 2 for me :)

Starseige Tribes

Cricket.

But if we're talking video games, I don't think it's been made yet ;)

S.t.a.l.k.e.r.

Gauntlet: Dark Legacy on PS2

Game of Thrones

hitman.

takedown: red sabre

HALF LIFE 3

Terraria and Half Life (all of them)

That I've played, Half Life 2. Dragon Quest 8 is a very close runner up as it does everything perfectly, but I generally prefer shooters over anything else.

Halo <3 but i like many other games :D

Final Fantasy X of course :3

It's Beyond Good and Evil!!