The STALKER games, especially the first one because of exploring the environment for the first time, Bioshock again just got immersed in the world, though I had to take a break half way through because the game play got a bit repetitive, and then of course the whole Half-Life series. Portal left me feeling actually ecstatic for a while after which is a bit different to wanting more, eventually though I gave in to realising playing again would never have the same effect again.