I personally loved the magic system in Elder Scrolls: Morrowind. It gave me as a player a lot of freedom.
You bought basic spells which were based on fire damage, levitation, health improvement and such. Once you own them you could combine them in any way you wanted. You could apply them to yourself, by touch or by fiering a blob at some target. It was also possible to embed them into items. Making more powerful spells and items was more expensive and could result in out of level spells (which you could not use). In Elder Scrolls: Oblivion and Skyrim they kept some of the customization tools, but they were not as free as the Morrowind system. This was because of balance issues. In Morrowind, I made a ring that would practically kill everything by touch. The end-of-game creatures and final boss took perhaps 2-3 hits, but still: OP!
How it looked: