A 32-bit dual screen handheld with both screens working in tandem.
Visual Boy Advance may be the best Gameboy Advance emulator in our opinion but for the Nintendo DS we have to go with NO$GBA. Most of the Nintendo DS emulators are in early stages and out of those this is your best bet.
A very close second to NO$GBA and was also the first Nintendo DS emulator able to run some commercial games at a playable level. Its worth the mention becuase some games may work on NO$GBA and some may work on DeSmuME. Both good emulators for what can currently be done.