Genesis- decent.. no frills emu. Another emulator is in development.
Snes- Snes9x runs quite well espescially if overclocked. Other emulators may be in development. Rumours abound that we may see a good release before Christmas.
GBA- About a million miles away from good emulation. Will probably never see good emulation
GBC- 3 good released emulators. Really good at the moment- VBA, fGB and the other one, whose name escapes me.
SMS- brilliant- amazing compatability- good speed and great options from fSMS.
Other emulators include Castaway which is an amazing Atari ST emulator. There are so many classic st games which make this emu a must. Also NES emulation is brilliant with Little John and NesterGP.
The problem with GBA game that could work on VGBA are the rpg's and they mostly have +8mb rom size, so some buffering technique would have to be used in the emulator.
and, Rick Dangerous 1 and 2, Prince of Persia, Gods, Entombed, Vroom, Sim City etc. The ST is one of the greatest games machines ever made and it's emulated perfectly!!