Unicorn Overlord is my current GOTY pick, although that might change after I play a bunch of contenders before the end of the year.
The most fun I had this year, though, was with two Spike Chunsoft games: Natsu-Mon and Bakeru. Natsu-Mon really left a deep imprint on my heart, and Bakeru provides the sort of 3D action-platforming and collecting I love.