For GB - that's pokemon gen 1 games -
THIS rocks. Sure, it's monochrome and stuff, but that's how these games were released. Absolutely perfect.
For GBA - that's like pokemon gen 2-3 -, most of the emulators either don't even run the programs, or distort the sound on peak level.
THIS is what I found the best of the bunch.
For NDS - pokemon gen 5 - use
THIS. Again, if you pick something else, you'll face sound-distortion. If you are lucky. If you are not - agn, won't even start.