Well it says on my profile Metroid Prime, but that's only because I've never finished Majora's Mask or Rayman Legends
Metroid Prime, Dark Souls, Banjo Kazooie, Majora's Mask, and Rayman Legends are the pinnacle of single player games from what I've played.
I have a strong feeling that many other franchises would be competing for number #1 ... if I played them