"Which RPG besides D&D has the best magic system? Give details."
Who knew I'd be talking about the Iron Kingdoms again so soon? Or maybe not so soon. This pre-posting stuff is starting to give me an anachronistic viewpoint on life. Regardless, currently my favorite magic system is utilized in Iron Kingdoms, and I favor it over D&D because it eschews the traditional concept of spell failure percentages when wearing heavy armor.
In D&D, wearing anything heavier than basically a tee shirt gave you a chance for your spell to fizzle in the casting of it, probably to prevent casters from walking around wearing plate mail. Iron Kingdoms says to hell with that and decks their Warcasters out in powered plate mail, carrying swords and guns and sledgehammers, and proceeds to beat the everloving piss out of the opposing Warcaster, who is also decked out similarly. In Iron Kingdoms, magic doesn't have to define who you are. You can be a sniper and still cast spells. You can be a warcaster and a Stormblade. This is a setting that looks at the Rule of Cool and asks what else it has up its sleeve, because while giant steam-powered robots are cool and all, what about people who want horse-drawn tanks?
There's also the lore around magic, which involves certain gods disappearing, certain races being really pissed at others and blaming them for the disappearance of their gods, and certain races who use necromancy to cross oceans and enslave entire civilizations.
So, to summarize, in Iron Kingdoms, you can use magic to cast spells, telepathically link with steam-powered automatons, infuse bullets with magic spells, and inscribe non-magical weapons and armor with slottable rune magic (or the souls of your victims, if that sort of thing's your bag, baby.)
God I love this system.