My Favourite emulators at the moment are (Probably in order of use - although WinUAE is only top because of the WHDLoad packs, not from playing games).
WinUAE (Can't find any info on this one...)
Mameox128 Plus! (Xbox Arcade emulator, plays all the oldies perfectly!)
FBA-XXX Pro (Xbox CPS1/CPS2/NeoGeo Emulator) - One of the best emulators on the xbox. I'd get this just for the Metal Slug games!
Zsnexbox (Xbox Snes emulator) plays everything I've thrown at it, inc. 4 player support & the NEW Zelda 3! (Parallel Worlds = awesome)
NeoGenesis (Xbox Megadrive emulator)
Mednafenx PCE (Xbox PC-Engine/Turbo-Gfx16 emulator)
WinUAEX (Xbox Amiga emulator) For when I can't be arsed to drag the Amiga out.
MekaX (Xbox MS/GG emulator)
Surreal64 xxx (Xbox N64 emulator) Plays 'some' games absolutely perfect.
NeoPopX (Xbox NeoGeo-Pocket Emulator) Only played Sonic on this but it's nice
I shall be grabbing a GBA emulator once I've got the above all working exactly as I want