For this list I am goning to ignore Aurelion Sol and Targonian Aspects, as they are not actual mages but rather cosmic entities. So let's begin.
He is very knowledgble because of his demon. Speaking of his demon, it's the source of his power, and has allowed him to take control over the empire and defeat Darkwill.
This guy is said to have influenced the affairs of Noxus since its earliest days. He is a master hemomancer, allowing him to control blood and have mastery over blood spells, as well as manipulating others.
Her speciality in magic is manipulation and deceive, as well as making illusions. Has been around since Mordekaiser's reign, and is almost as old as Ryze (Rune Wars were before Mordekaiser, since every nation was destroyed as a state during them and Noxus still exists, Noxus was right after Mordekaiser, thus the logical events would mean Mordekaiser took advantage of the Rune Wars to become a tyrant after they ended.) Her age as well as her acces to magical artifacts and ancient knowledge makes her very powerfull.
One of the most powerfull and the most ancient Void-Born on runeterra. Vel'Koz uses void magic in the form of plasma to disintegrate his targets to research them. He is also the most intelligent Voidborn, and given his age his knowledge is one of his most powerfull weapons.
This guy is being powered by a World Rune and he was able to hold his own against Ryze for a while. His fire magic is very powerfull being capable of casually melting through a large building as well as having acces to a wide vareity of fire spells. Ah and he was Ryze's apprentice before he got coruppted by a World Rune.
She was given the capability of manipulating Dark Ice, a corupted version of True Ice which is unmeltable, by the Watchers themselves.. She was around for a long time so her knowledge and experience of magic is great.
A lich from the Shadow Isles, Karthus is able to utlise Black Mist magic and control the souls of the dead. His necromancy and experience gained from his long-live is not to be underestimated.
A very powerfull mage, her color story that was realeased by accident and it's gonna get realeased again in the near future shows her telekinessis being extremly powerfull, being able to hold in air an entire building. Her Dark Spheres can go through a normal human like it's nothing, and her raw power is incredible.
Xerath is an former magus, now Ascended. His magic was buffed a lot by his Ascension and he was able to beat Nasus in a fight in the story "Bloodline", as well as stalemating Nasus and Renekton while fighting them both. He is capable of magic energy blasts that stun his target, energy beams, firing white fire comets, and much more.
This guy is old asf, and knows a ton of spells. His exposure to the magic of the World Runes heavily modified his appearence and buffed his magic. From several quotes from his stories we know that a part of the World Runes energy flows through Ryze's entire body, causing his tatoos to glow when he is using magic as well as his eyes when he goes all out. We have seen Ryze one-shotting a high-tier voidborn when he got serious, gaining the uper-hand against Brand in his comic without any struggle, whom I may add is being powered by a World Rune and Ryze defeated him without going full-out, since his eyes didn't glow and they glow only when he uses his magic all-out or uses high-tier spells.
Tell me your thoughts about my list and if you think I got the order right. Give arguments pls.
Sorry for my bad english.