Inc. All rights reserved. This part, at least, is simple. All the voices must form a good melody. This is usually the most difficult part though. These are not harmonic intervals, but intervals by which the melody moves.
Some intervals are simply more melodic than others.
Clearly the best and most widely used are whole tone and a minor third. Halftone and a fourth and fifth follow up closely. Fifth and octave are sometimes not melodic at all - more akin to repeating the same note. Sixth is a powerful melodic interval, best used as an effect and usually up. The important factor here is the use of your right brain and your ears.