I figure the best solution is just heaven versus Hell -- and not really in a "Good versus Evil" way. Rather, they are the powers on which the fate of the world rests and they are at war. Humans might think of Heaven as Good and Hell as Evil but that's just a mad attempt to make sense of a struggle so far beyond mankind it can't really be measured. Sort of the way Diablo looks at it.This is why I cannot get behind just 3, especially within the western fantasy tradition that is D&D. Moorecock is absolutely part of that tradition.
Good vs Evil
Law vs Chaos
I dont think Neutral needs to be a thing if I think about it.