Dragon's Dogma has to be near the top of the list for me...if not number one.

Loom

Tyranny had a fun system where you could create custom spells based around combining a Core Sigil with Expression, Accent, and Enhancement Sigils to modify the spell's behavior. So you could have a Fire core sigil, combined with an AoE expression, fast cooldown accent, and a bleed enhancement. Of course the spells have a cost attached to them so you couldn't have your mages casting ridiculously powerful spells on rotation until you ran out of magic, but you could pop it off once or twice then fall back on weaker, faster spells. Unfortunately like with most flexible magic systems like this, mage characters are overpowered as hell, as long as you have your party tank camping a chokepoint.

Divinity: Original Sin 2 has a pretty robust magic system. The chaining status effects are fun and there are multiple ways a player can turn a fight into their advantage.

The spellmaking in oblivion was pretty great too, you could make multiple spells which applied weakness to magic and to specific elements in enemies, and since they were applied by different spells they would multiply each others' effects. Then you hit the enemy with a damaging spell and annihilate them.

I never saw it mentioned anywhere, but for me, hiring enemies with sufficiently powerful magic would ragdoll them, a bit like if you hit a corpse or a paralyzed enemy. So doing a 4-3-2-1 combo, 3 different spells that applied 100%weakness to magic and lightning, then hitting them with a lightning spell, would always send them flying. Sometimes it would be like they just vanished because the amount of damage applied by the spell threw them so forcefully.

Dishonored series. Coolest system I've ever played - teleportation, possession, and some fascinating level design in the sequel where you jump between times with magic. So cool, in fact, that I've had dreams that I have the Dishonored powers.