i would argue that aegwynn is actually the greatest azerothian spellcaster.
she fought medivh without her guardian powers and it was actually not an easy fight for medivh. medivh had not only the powers of the guardian, but whatever power he got from sargeras' possession as well.
so i think it's like this: aegwynn with guardian power > medivh with sargeras + guardian power > azshara >= aegwynn without guardian power > medivh with guardian power