I know some of these have been mentioned here before, But I'll go ahead and list the emulators I mainly use here. I remember having to press a certain key just to go back to the ROM loading window.

Stellar - An Atari 2600 emulator which works smoothly but is entirely minimal (as in, no menu bar at the top).

People said that ZSNES was better but, I had a hard time trying to even load a ROM with it.

Snes9X - It might not be the best of all SNES emulators but, I felt more comfortable with it.

It is one emulator I would highly recommend.

Kega Fusion - Good support for all of Sega's systems, from the MegaDrive/Genesis to the Master System.

VisualBoyAdvance - Pretty much everyone uses for all Gameboy systems, and was the second most-used of all.

I think I can recall a Chinese version of this too.

VirtuaNES - Another NES emulator, but this seemed better for the following reasons: the ability to quick save/load during gameplay and, has support for unusual mappers each time it is updated, mostly for these weird bootleg games floating about.

FCEUltra - The NES emulator I used most often, and one of the best I had used.

When I used to do emulation gaming years ago, I used the following:

Shoebill mac emulator