Anasterian held his own against Arthas longer than anyone, and only lost because he was old and frail. On the purely magical side of things, he single-handedly destroyed the enormous bridge of ice that Arthas was creating to cross the large channel between Silvermoon and Quel'Danas. He did that with a huge crazy powerful fireball type thing.
Rommath rebuilt Silvermoon and reclaimed Eversong Woods from the Scourge overnight with just a handful of magi. He was Kael'thas' right-hand man from the Scourge invasion up until Kael'thas' betrayal in BC. And Rommath also discovered how to channel stolen Light from M'uru. I'd say that's pretty advanced stuff.
Aethas held up a huge bubble shield around Dalaran in the Mage manga to protect the city from attacking Blue Dragonflight, shown to be on the same level as Rhonin and Modera, who were doing the same.
Jaina has been the one in charge of a bunch of other humans. K, I guess that counts as being a master mage somehow? She's set fire to a giant scarecrow wickerman thing outside Lordaeron during Hallow's End as a teenager. She was a student of Antonidas, though you consider Rommath to not be proficient despite being highly valued by Kael'thas, so her being Antonidas' pupil doesn't count either then. She can teleport and make portals like all mages can. She can throw a spell in a fiery burst of anger, then have it be lazily waved off by Thalen Songweaver like it was nothing. She can steal a very powerful artifact and have it do all her work for her, only to be held back by Thrall. She can violate her neutrality by warding an Alliance base against Horde infiltration and personally kill any Horde who tried to get around her wards to get the WMD she was protecting for the Alliance. She can apparently walk into a room, murder 6 master Blood Elven archmagi bodyguards (meaning they're specifically focused in magical combat) before any of them can even fire off one spell at her or her slow-casting water elemental, so I guess that's something we just have to pretend makes sense because Blizzard tells us to. She can teleport bewildered innocent Sunreaver civilians just beaten senseless by a rabid Silver Covenant brute squad into her magical prison, so that's impressive, I guess. She can steal Lei Shen's magic after he's dead and infuse it into the already very powerful staff of her former instructor.
But I guess fury of a 30-year old woman who considers herself scorned because someone she'd been helping someone else kill for years before attacking on her own for a year finally had enough and wiped her legitimate military target off the map trumps hundreds/thousands of years of magical study.
I may be mistaken, but isn't Khadgar only as strong as he is because Medivh transferred some of his power to him during the Life Drain in WC1 a la Voldemort making Harry a horcrux? I was under the impression that that's the only reason he was so powerful, enough so to lead the Alliance Expedition to Outland despite being like, what, 20 at the time, just in an older body that was given lots of magic power from Medivh by accident?
He's also got Atiesh though, so that's a big boost, I'm sure.
I don't think Jaina being the leader of a non-magical nation necessarily adds any points in the magic power category. Nor does being leader of Dalaran after a trumped up "prophecy" that Blizzard threw out the window after only, what, an in-lore year? Year and a half?