Gameboy Advance

I still have this one lying around somewhere. The only games I really had for it were Mario Kart & Golden Sun, but they were awesome.
Its formfactor was a bit too small for my big European hands, especially the L & R buttons: they were literally a pain to use.
In retrospect, this particular handheld just might be classified as the predecessor of the new Wii U Controller since you could connect the GBA to the GameCube and us it as a controller with extra gameplay on the GBA's screen.