Retrocopy in future will be most accurate, also you can adjust sound to your sound system at 100% accurate, Kega Fusion have typical audio settings and you can't adjust anything for sound
Another emulators requires less of PC power, but they can't emulate all aspects of real things such as real pallete, color dots at color borders, muffled sound of real TV's, they can't run many games at once.
Kega Fusion have experience of 12 years, and Mega Drive in Retrocopy just few months, now we look at early stages of Gens and Regen from initial versions they can't do anything with experience few months.
On early stages of Mega Drive emulation, Retrocopy showed exelent result, new releases of Retrocopy will be more and more cool