Not an easy question and I don't actually write much abc. For a while, I've been mostly keeping a site that uses abc running rather than using much of it (or any notation/music) myself.
I could come down on the side of abcm2ps as these days and with the absence of anything beyond dafts for some time and no standard dealing with w:, I take it as the closest to a standard we have.
For use at folkinfo, I'd not like to be without abcm2ps or the abcmidi (we use abc2abc as well as the abc2mid) package and feel a need both to produce a "sheet music" representation and a playable midi file.
For personal use, I like something that shows the dots and easily allows playback (I have to hear things). I'd say BarFly is the most complete implementation I've have tried but I don't have a Mac and have not had that great a success dabbling with emulators. I think 5LineSkink holds the greatest hope for me and really must (I did start looking a while a go but it got forgotten about with other things going on in life...) at the new release. It's has the potential to be my first choice for writing abc.