Obliviate

This spell throughout the entire series would be one of my favorite spells. The idea of erasing memories from an individual has always been very intriguing to me simply because I find when one has such an ability, one has great power amongst the other.

What are memories? It's a recollection. A remembrance of the past. To be able to rob a person of their memory is almost like taking away their past. I find that a person's past is a vital part of them. Our pasts make us who we are, they help shape the people we are meant to become. Memories make up part of the essence of a person.  Certain events, certain people and certain places all gradually define who we are and when one of those three alters, you'll find a different person. The difference might be subtle, in some cases almost colossal. But there is difference nonetheless.

This spell has the capability to change an individual's mind, body and soul. It won't torture you mercilessly as Crucio, it won't kill you like Avada Kedavra and it certainly won't transform animals into water goblets like Fera Verto. However, I think one of the biggest damage (apart from killing them, obviously) you can do to a person is take away a part of them that makes them who they are. Which is exactly why this is my favorite spell.