Portal, for the reason the author has stated, and due to it being sheer awesome, but also Shadow of the COlossus. A massive open world, the means to get around fairly quickly, Only twelve gigantic enemies, and the means to find them. Plot only progresses after ever four kills, and gets more and more mysterious as the game goes on. I fucking love this game, and I have never played any game like it, no, not even ICO.