It's about to, Guild Wars 2 isn't that far off! 
In all seriousness, I have a hard time having favorite games for long, simply because they diminish over time after I've played them, and it's not so much the games themselves, but rather qualities in them that I look for. Games like Heavy Rain and Mass Effect 2 are at the top of my favorites list right now, mainly because of they do well. Bending stories to make them fit your experience, and being very exciting and gripping at the same time.
The next game that will come out and challenge for that position will be L.A. Noire (with the investigation gameplay) and following it will be either Guild Wars 2, Mass Effect 3 or Uncharted 3. I don't know which one will come first, but fortunately the focus on story in the game industry is big at the moment and means my favorite games don't take many years to get pushed down slowly. 
