I dabbled a bit in demonology when I was just starting out. I worked with a few entities, eventually landed on Lilitu (Lilith), but she wasn't my matron per se, my relationship with her was more as an intimate friend and a mentor. I called on her for guidance, not rulings, and I had no call to strict loyalty to her. Looking into comparative mythology and my shadow, I realised that Lilitu represented that darker side that I had repressed, a soft, alluring, yet deadly feminine force. The femme fatale, the ***** of Babylon, Pandora.
For my most successful spell, it was a hex, but I know that some don't like dealing with those things, so I won't elaborate any further. Excluding that, it was a transformation ritual, at the time, a close friend of mine wasn't in the best place mentally (anxiety and depression) and, knowing what I do, she asked me if I could do anything to help. I spoke with her first, asked her if therapy or some other alternative was an option, which it wasn't. I taught her some of the basics of shadow work, as I believed it to be a major source of her problems, and I then conducted a ritual (that I had made up) with the intent of trying to promote conscious integration. She kept up the shadow work, and over the next few months I saw (and she reported) her problems, especially her anxiety, clear up considerably. She wasn't "fixed" in any sense, but it had made it manageable for her, and caused a major inflection point.
I'd make a distinction between my most successful spells and my most successful magick more generally. Spells are the ritualistic elements, the literal ritual I conducted, whereas magick is more about the directing of those forces more generally. Manipulation, relationships, the life you live, beyond their obvious, external (exoteric) shells, they cause hidden, internal (esoteric) changes, and are magickal in nature. I don't believe in some universal revenge principle like karma, but if you're manipulating to get what you want, the (in the easiest terms to use) "negative" energy you use when manipulating, you're surrounding yourself with it and so it's likely going to leak back to you. If you hurt people, hurt people hurt people, and you very well may end up being their target. The sorcery you are doing is unwise because it can threaten to disturb your balance. Also, all it takes is a gust of wind for your house of cards to come crashing down.
Went on a bit of a tangent, but hope you enjoyed it regardless
