Almost any Monster raising game. Like Monster Rancher Advance, Pokemon, and Sonic Adventure 2 Battle (the chao part ). Also I got pretty hooked on Dissidia, especially since I was stuck in a hospital with it for 11 days -_-'. But honestly the three games I've been the most addicted to would have to be either Sonic game with Chao, Pokémon (all...) and Fire Emblem. (the first GBA one)