I'd have to go with Centaur. It's not a particularly special spell, but it kickstarted my life as an all-offense life wizard. Lifeblade, Life Shield, Centaur (+robe and hat bonuses)... I just killed any Marleybone enemy.
Frozen armor because you can spam zero pip buffs and shields, get to max pips, put up the shield and then do what you need to for your attack. This strategy has worked countless times against so many bosses (especially the ones you have to solo).