For Genesis it's definitely Gens for me, which has brilliant compatibility and also works with 32x and SegaCD games.
For Game Gear you could try Meka, Dega or FreezeSMS. These also work for the Master System too.
For Virtualboy, there aren't that many to choose, so just download them all (there are about three altogether). Bare in mind that Virtualboy emulation is not exactly excellent.