My favourite genre has to be adventures, although it seems to be a genre that's falling out of favour at the moment.
Lots of PC magazines are signalling the end of adventure games (again). I remember when this first happened, in the days when text adventures were becoming obsolete, but then the genre evolved with the release of some of the best games ever, namely the lucasart games. I can't really can't see this genre dying, although I think it probably is in need of some new ideas.
Hard to pick just one games, but it'd be between:- Monkey Islands series, Simon the sorcerer, grim fandango, discworld noir, gabriel knight series, broken sword.
Lately I've found myself converted to rpg's, namely the final fantasy series and the parasite eve games.