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As I stated in Best emulators non-psx for 3.10oe:
I've had a lot of fun with snes9xTYL, I'd really love to see someone pick it up and polish it up a bit, really some good FX Chip support and I'd be set! Starfox 1 and 2!
StrmnNrmn has done AMAZING things with Daedalus but honestly I still don't find it quite playable yet... (so close)
I suppose DGEN is about the best MD/Genesis emulator but I find it to be rather glitchy and prone to slow-down, beggars can't be choosers though!
I don't have much experience with GB/GBC/GBA emulators (since I still own all the games I'd be interested in emulating), perhaps someone else can chime in there...*
And lastly I'm sure CPS1PSP and CPS2PSP would be great for a bunch of the Capcom arcade classics but I haven't sat down to figure out how to deal with the multi-piece roms yet.
*Regarding GBA emulation you'll want to look into gpSP by our namesake Exophase and for GB/GBC get RIN.