What's your favorite game of all-time?

Title of the topic pretty much speaks for itself. What's your favorite game of all-time? Perhaps you also can add on which console you played it and why it's your favorite game.

My personal favorite is Portal 2 from Valve, I got it on the PS3 and PC (since it came with a free PC copy). I'll admit that the graphics weren't that impressive, but still impressive enough for a game engine that's relatively old. I absolutely loved the game characters and especially the story. The different testchambers from different era's were amazingly fun, and very often it took a while to crack the level, but it was at the perfect level of difficulty. GLaDOS was still as funny as hell, and the ending was surprising, and absolutely perfect. I did not expect the turret opera at all. I do hope they'll make a Portal 3, but since Valve is still learning to count to 3 I'm not quite sure. Pity there was no cake in Portal 2.