In hindsight I regret giving my mage Earthquake & Petrify.
Earthquake looked cool, but it never seemed to do what it was supposed to do, i.e. stop enemies in their tracks. In the endgame, I discovered Grease always works when I gave it to Wynne. Combined with Fireball it's just silly powerful.
Imo it should've been possible to trade in a spell for another one each level up. This would've given the opportunity for the player to try out everything and discover what works and what doesn't.