That's a good question. So many games I really enjoyed this year.
Endless Ocean:Blue World, Tatsunoko Vs Capcom, Monster Hunter Tri, Metroid: Other M, Kirby's Epic Yarn, Goldeneye. All have gotten fantastic useage but I think I'm going to have to give the edge to:
Kirby's Epic Yarn
It's gotten so much play as I play with my daughter, my gf's son, my niece and nephew, etc., and when my testosterone is high enough I can afford to lose some, by myself. That one game has been played on so many different Wii's now I've lost count. It's such a great game for new and experienced gamers alike. It's quite challenging to get a great score all the way through but no matter how badly someone may suck, they can still finish the game and feel proud of themselves. Everyone who plays it, has a lot of fun with it (even if they don't like to admit it). Now if it wasn't just quite so kiddie...