As a PC, my all time favorite is the cleric spell Flamestrike. That spell is frickin awesome. Half of it is fire damage and half of it is divine damage which bypasses just about any damage reduction.
As a DM, I’m overly fond of Silence and Greater Dispel Magic. Nothing like making PC spellcasters epically frustrated by not being able to cast most of their spells and for all the party members to lose their buffs and have their magic items effectively neutralized.
I know. I’m evil.