Once you've obtained a 90 skill ranking in any school of magic in The Elder Scrolls V: Skyrim, you're able to go questing for the master level spells. What are these? Why, some of the best spells in the game! Now, you won't get any achievements for these, but you will have some of the best spells ever.
If I were to try to explain how to get these, I'd botch it up very quickly. However, the lovely pms00 from YouTube has a great set of tutorials for each school of magic. You can see those below. Right now, I'm wondering why I never put in a lot of time to upgrade my magicka skills. Guess I'll have to make a new character, then!