Fusion if you're not too attached to cycle accurate emulation.
Otherwise Blastem, Genesis plus GX on retroarch. Blastem is very fast and cycle accurate, but it doesn't support Mega CD or 32X. Genesis plus GX is also very accurate, its sound engine might be the best of all emulators, it also does Mega CD but no 32X.
And lastly Ares, the most demanding of them all, very good accuracy but Blastem and Genesis plus GX are just as good and run on any PC that isn't 20 years old, it does Mega CD too and it is also the best 32X emulator (the only one that can run the DOOM remake that combines Mega CD and 32X).
I haven't tried JGenesis yet, but I'll give it a go.