What is your favorite game?

[Guest]

Ok, if we're going to have an off topic discussion, then why not tell everyone what your favorite games are, besides Penumbra, and what you like about it. I don't really have much to add here myself, because I don't go looking to play a lot of games, but it seems like if we talk about what makes a game good then we will have someplace to start thinking about how Penumbra could be improved, or what would make a good mod.

I'm especially interested in how other games use features that I've only seen in Penumbra, like physics and an inventory screen in a game with a first person viewpoint. I'd like to hear about games with really challenging puzzles too.