This could be a spell that encapsulates the series' magic system, or just a notable standout that stuck with you, or whatever.

Favorite - Giga Slave from The Slayers. It's a spell the main character cobbled together from an ultimate spell invoking a demon god, instead changing the incantation to instead invoke an entity greater than that. It 100% destroys its target if it connects, but requires a level of control and focus or it would destroy the world outright. It's cool to me in that it was an invented spell and you don't often get to see active creativity in a magic system. It also creates a cool moral question for the characters about using such a spell. The magic system of Slayers is just really cool like that.

Least Favorite - Avada Kedavra from Harry Potter. It's an instant death spell and anyone hit by it dies instantly with no chance of resuscitation or revival. It's illegal in the setting, but it's only ever used by the bad guys. There's no history to the spell, no counter measure, and it's considered just evil. Meanwhile there are other spells that are technically lethal on hit, but are perfectly fine to use against an opponent during mortal combat. It's interesting because on the surface, it invites moral questions. A quick and painless death spell that has little chance for collateral damage if it connects.