Portal

Every word you may have read about this relatively short, exceedingly funny and astonishingly perfect game is true. I would never have imagined that my favorite game of the year would be a physics based first person puzzle game, but it's so much more and does everything right from the difficulty curve to the character and feel of its environments and its brilliant antagonist.
Also: When was the last time a game rewarded you for winning with cake and a song (that still keeps me grinning and happily giggling when I think about it)?