Happy birthday Gameboy!
Personally, as great of a handheld the Gameboy was, the most innovative kind was the Gameboy Advance SP. It was extremely portable with it's clamshell design, built-in back light so you can finally play in the dark for the very first time with real color, and built in lithium battery. It just made the original Gameboy Advance look like a piece of garbage lol. Really changed the handheld world for me. Obviously it was just a matter of time and technology that made this happen but still :D