Not only am I a little late to the freakin' party, I have absolutely no idea what my favorite game is. Or even genre! So I'm gonna try to ad lib some stupid "I can't pick one favorite" crap.
Where to start...
I don't really have a favorite game, because I like different games for different reasons. I like Oblivion for the mass customization and role-playing, and I like (for lack of a better example) Halo for its fast paced combat. I have noticed, however, the more of different types of games I play, the easier it is to get the brain juices flowing for different gameplay aspects. Fast paced games get my combat system ideas flowing, as where RPGs get my customization story gears cranking (for unknown reasons, the less story a game has, the more story stuff I think of). Holy crap... It's like a freakin' game. Playing other games to raise my skill level with conceiving different concepts... Now I sound like a retard.