The M3 is considered the "best" flash device for the Nintendo DS right now, at least by many people. I believe it works in the GBA as well, but I am not sure.
With the M3, you can play GBA and DS games, as well as some emulators as well. It also uses SD or CF cards (depending on the model you buy), so it's much cheaper to add more memory when compared to other flash cards. The M3 itself, however, is a bit expensive.... but worth it IMO.