Aegwynn - She is the most powerful Guardian of Tirisfal, serving in her role for a thousand years - she fought and almost defeated Sargeras's manifestation not once but twice - the second time was when she was without any Guardian abilities, and she was still apparently single-handedly winning her fight with Medivh, being directly helped by Sargeras himself, at Karazhan. She was able to easily elude the Kirin Tor for decades, as well as the best of their Tirisgarde. She was the one who sealed the Tomb of Sargeras, and was able to subdue Aluneth, which she used to create Karazhan, as well as wielding Atiesh, Greatstaff of the Guardian.
Azshara - Arguably the most powerful and renowned night elf ever to live. Greatest of the Highborne rulers. She has been described as the most powerful sorceress in existence - to have ever lived. Destroyed Pillar of Creation easily (Tidestone of Golganneth). During the War of the Ancients, Mannoroth realized she was actually far stronger than he was - that only Archimonde and Sargeras provided superior to her, and this was 10,000 years ago, when she was much less experienced. Aluneth stated that naga wielded the arcane without finesse - but there was a time Azshara's people apparently "had the potential to rival the titans". Although Azshara today is without the Well of Eternity - she does have access to considerable Void abilities along with her arcane ones.
Who would most likely win in a real fight if Aegwynn, when she was serving as Guardian of Tirisfal, had fought Azshara, in some random location where neither of them have the advantage, without any assistance?