Definitely Arcane/Divine. In older editions, cleric spells were significantly less powerful than arcane spells. This is no longer the case, and there are very few spells left which haven't crossed the divide. I'm not saying, however, that each class should have exactly the same spell list.
Create an extensive list of "Basic Magic" -- spells which anyone with magical training can
learn.
Then to distinguish the spell lists of the different classes, distribute the more unusual, exotic, and powerful spells into domains, spheres, schools, and so on. Each class would automatically grant access to a certain number of these, but a character willing to spend feats and time on the effort should be able to learn magic from a style not in his class.
Furthermore, each class should have a different way of casting magic. Leave V,S,M for wizards, make bards V only, introduce other modes like C(oncentration), R(itual), etc for other classes.
(Yes, I know I'm cribbing from Arcana Unearthed here--but I think this would be the easiest way to do something similar in D&D. Domains were one of the great ideas of 3rd edition.)
