Besides D&D?! What do you mean besides D&D! D&D has a crappy magic system which I've always hated, because it's crappy. Also, I hate it. And did I mention it's crappy? I think that D&D (except for 4E) has probably the WORST magic system of any RPG. The exception is 4E. 4E pretty much fixed D&D's crappy magic system--so of course they didn't continue with it in 5E. So I would say D&D 4E, except that this challenge question is for non-D&D systems.
Overall I've always liked the magic system in Chivalry and Sorcery. It's based on logical concepts, has a built-in skill system (you should have to roll to successfully cast!), allows you to really specialize in a type of magic, has a wonderfully detailed magic item enchanting system, and loads of magic user types to choose from--all in the main book. Actually the magic rules in C&S were the best part of the rules. Playing a magic user in C&S was way more fun that playing one in other games. It had a really detailed, hands-on feel. Magic user players really worked on improving their spells, sought out unusual magical materials for making items out of, and constantly wanted some time to work on their magic stuff. And all of that flowed organically from the way the rules were written.