Portal is a pretty obvious choice. Though I've been known to have 30 second games of Solitaire... Talk about a terrible draw... (play Vegas style and you'll come across it too). I think next to that, I would have to say Prey. So much potential there, but it was all marred by fluff. Sure the game had a huge scale, but when it boils down to it, you didn't really DO much. You spend about half the game flying around in some bizzaro spaceship thing, and the other half pummeling things with a wrench. Bit like Bioshock, only with nausea inducing gravity physics and semi-revolutionary portal technology which more or less inspired that Companion Cube laiden gem of a game mentioned earlier in my post.