I have so many favorites, but if I had to pick only one, it would be CastleVania: Symphony of the Night. My favorite CastleVania game of all time! Even after exploring the castle to 200.6%, I still like going back and picking up rare items and maxing out the familiars. The art style of the game is outstanding!
Lately, I've been seriously addicted to Gin Rummy, playing it hours on end. I think I may grow tired of it soon though since the online community for it is growing thin.