Based on the fact I played it the more than any other Nintendo system, I think mine was probably the Gameboy Advance. I got super addicted to the Pokémon (Ruby Red maybe?) that was on it, and I played it for a really long time. I liked the clamshell model a lot too. The DS is a close runners up for me. I would kind of like to have a 3DS, but I haven't felt like spending the money yet.