I'm not talking about FAVORITE here because "pure fun" and "all around favorite" are two different things. What game do you have that has you grinning with glee when you play it? Or which, on instant thought, do you think "That's a really fun game?"
For me, I'd have to say it's Rayman Raving Rabbits. The game is just hilarious and ridiculously fun. The bunny antics are priceless and the imagination in the game design is just great.