my recommendations: all of them!

I say that for 2 reasons; first I think it is *amazing* to have so many different platforms all under 1 roof. It's like having a little console museum right in your living room. Even odyssey 2 gets occasional playtime at my house, and I think it's extremely cool to go through console history in an afternoon, starting at the O2, then working through them all the way up to the PS1 and N64!
The 2nd reason I say that is that there really isn't too much overlap between emulators. There are 2 SNES emulators, but only 1 of them is in active development.
IIRC, all other duplicate emulators were simply superseded by one another, since there is essentially only 2 people working on the entire emulator scene, and 1 of them ported 95% of all the emulators.
Being a bit of a purist, I shy away from the handheld emulators, as I dont think they should be played on a TV set. I have every available console emulator installed though, and I'm in heaven.
This is a nice guide as to what to get:
http://forums.xbox-scene.com/index.php?showtopic=386399and then for everything else you'd need, screenshots and configs and everything else, pop over to that
http://xbox.nugnugnug.com site, which I happen to be the owner of. (thanks for the plug walls83!)

-jeff!