I hafta agree with Lucas.
Seeing the Mame-X emulator in action is what made me get my Xbox modded in the first place, and hearing that there was a Super Nintendo emulator that played games damn near flawlessly sealed the deal. After playing with SNES emulator for MONTHS, I have no problems saying that its my favorite emulator. Mame X follows right behind, being able to play most of my favorite Neo Geo games, Smash TV, Dig Dug (yes, Dig Dug), and R-Type 2 is just mint.
Thing is, Final Burn might soon surpass Mame as my arcade emulator of choice. I'm a Fighter and Shooter nut, and Final Burn plays Street Fighter Alpha 3, Vampire Savior, Guwange (you HAVE to play this if you like shooters), Dimahoo, and Esp Rade wonderfully. And now that I'm starting to get into Marvel vs. Capcom, Mars Matrix, Dangon Feveron, and Dungeons n' Dragons (ok so I'm a bit late - sue me), its like christmas all over again, with all these new games to play.
But SNES is still my favorite. It runs games almost perfectly, there's a lot of RPG games that have been translated into english that I'm now able to try, and there's many, many games that I've played long time ago that I never beaten (R-Type 3, Castlevania 4), that I'm able to run through now. With the ability to have savestates, game genie codes, a really nice, functional interface, and adjustable speed settings (YAY! I can skip cutscenes in RPGS!), its simply one of the best emulators on Xbox.
Now if we could only get the Genesis (my favorite 16-bit console) emulator to run as well...