18. Portal (PC, also on PS3 & 360)
This was pure genius. Only a few hours long with a few bonus maps, but everything was sheer brilliance. The innovative portal gun, some inventive and mind bending physics puzzles and wonderfully funny. I played this on my own and went through the whole thing in a single sitting. A few weeks later I spent a few hours of a gaming session with some mates going through the campaign and simply laughing at their reactions. Pure gaming genius.