My choice for game of the year is Oblivion without a shadow of a doubt!
In my 20 years of gaming i've played many fantastic games, some really good games, quite a few average games and a lot of poor games.
None of these has ever immersed me and entertained me like Oblivion has.
I spent 302 hours completing the game and when I say completing I mean it. I've done every quest, side quest, shrine, cave, cavern and ruin. The game just seems to grab you and not let you go.
The graphics are very good, the sound is excellent but the gameplay is awesome!
Gameplay is king and many developers/publishers are still to work this out, even after 20 years!
My honourable mention for this year is Shadowgrounds.
An old school top down shooter much like the classic Alien Breed.
It gets a mention simply because it's a fun game to play with a truck load of weapons and lots of aliens.