I know my username's named after a Mystery Dungeon game, but I actually like Pokemon Sapphire as my favorite, since it's what introduced me to Pokemon in the first place.
For non-Pokemon favorites, Skyrim, Doom II, Starcraft 2, and Age of Mythology are also among my favorite games.
I'm a huge fan of mods, so once I've finished with a vanilla game, I love to try out stuff like custom maps and scenarios for that game. The classic Doom games are amazing for this and, though I've only had them for a year or two, I could conceivably see myself playing them on and off for the rest of my life. They've had a huge modding community that's older than I am, so I don't think I'll ever run out of Doom maps to play.