The original GBA for me. An opinion based on nostalgia, because playing those game I was amazed coming from the Game Boy Pocket, and the games were all amazing and exciting, and I was at a point in life where nothing was troublesome.
The DS had a lot of great games (OSU!!!!) but my heart still lies with the GBA.
I remember playing Prince of Tennis, hecking fun! And also there was a spin-off game called Minna no ouji-sama, I played that music game to death! A DDR clone in mini format (the game was by Konami, so... not really a clone).