I already listed my 2012 favourites, but here are some special mentions...
Best game music: Gunlord (Dreamcast).
I've unfortunately not had a chance to play this obscure German indie game - which is a 2012 release - but based on the gameplay videos, it's the hands down winner of this category with its cool, electric old-school tunes.http://m.youtube.com/watch?v=0jhT8NjAKsw
Best visuals: tie between Journey / Mass Effect 3 / SSX / Assasin's Creed 3 (just my opinion, quite a few graphically fine games this year)
Best mobile game: Hill Climb Racing (silly, but addictive), Angry Birds Star Wars (even sillier, but apparently I can't stop playing it
Biggest positive surprises of 2012: The Walking Dead, for making the old adventure genre exciting again, Xcom for doing the same for turn-based strategy, and SSX for successfully rebooting a much-loved but faded franchise.
(Still) the most underrated game in 2012: NFS the Run. It's a 2011 game, but since I only bought it this year and was stunned by the awesomeness of the game, it deserves a mention. The PC version looks absolutely super-fantastic on ultra settings on a big screen and the very fast and sharp gameplay is bliss for fans of pedal to the metal arcade racers like OutRun and Daytona.