Just one, and why. None of this "oh there's so many I can't pick" BS. If you can't make up your mind don't bother. Any platform, any generation, any genre.
My favorite game of all time is Portal 2. It's by far the best-written game I've ever played, every inch of the game world is detailed and creative, the mechanics are clever and innovative, the game offers a ton of content with the additional co-op puzzles and it's all fantastic, there's not a flaw to be seen. Close runners-up were BioShock, Super Mario 64, Bastion, Ocarina of Time, Majora's Mask and Half-Life 2.
Mechwarrior Franchise it was way ahead of its time, and I grew up playing it on another note i honestly don't think the new mwo does the originals any justice
I don't think I could definitively say which game is my favourite I don't think I could even say top 10 I have so many games I regard as my favourites especially some I played as a kid.
That would've been my next runner up. I just hate that Tim and Chris disappeared and that Microsoft bought Rare because Rare was the biggest reason I played the NIntendo. They had such great games Even played the GBA version of Banjo Kazooie even though it made the storyline contradict itself