Lore Sjöberg grades the various hexes and curses in the Potter World.
Avada Kedavra (The Killing Curse)
It kills people, which is boring. If you have access to a wide selection of magic spells, just plain killing someone strikes me as a failure of the imagination. Any decent villain understands the concept of “a fate worse than death.” How about a magic spell that makes it so everyone on your block parks right in the middle of a curb section that can normally fit two cars? If some evil wizard threatened me with that I’d give up faster than you can say, “Harry Potter and the Pervasive Inconveniences.”