While I have an answer, I am afraid that is a matter of opinion. Thanks to how these spells are activated and function, how much fun you have with each spell will vary.  Personally, I consider Mystic Hands the best offensive mod to put into your arsenal. Swinging around like a magically enhanced King Kong and flinging enemies with massive magical hands is a truly tremendous experience.  Meanwhile, for a spell that overall rounds out your arsenal, I have to give it to the Healing Spell. It frees up those crucial slots for weapons instead of potions.  Magic Mods can be found on the Nexus for PC users and the Nomad Nexus for Headset users.