Well, the easiest form for me is SongWright, followed (in no particular order) by ABC, printed music, gif, tiff or jpeg files. A problem with MIDIs is dealing with initial measures that aren't complete; another is that of too-accurate recording of not-accurate-enough timing during recording. Single melody lines are desirable; so is a great deal of restraint in adding ornamentation.

Any format, though, is vastly preferable to npone at all.