I myself am torn between Mysticism and Illusion for my favorite, while my least favorite is by far Destruction.
Mysticism has some of the most useful spells in the game, intervention, reflect, spell absorbtion, absorb, and the rarely considered dispel. While Illusion brings a TON of utility to the table for any class, invisibility, silence, paralysis, sound, blind, demoralize and the convience of charm, night eye, light and others.
Destruction mainly does one thing, damage, and with the variety of enemies that have resistances it is a pain to find the right spell for every situation, also having a destruction spell reflected back at you is much more painful than say, an absorb health spell. Props to damage attribute though, as I love that mechanic.