Noir

Aincrad is the only interesting part of the series. Kirito is kirito all the way throughout. Aincrad as a world with the prospect of death is interesting.

Literally every other arc has a great secondary lead (Suguha, Sinon, and Mother's Rosario has Asuna outright taking the spotlight)

Aincrad had the best premise, yes, but it's not particularly anything new. Meanwhile the other arcs have huge improvements to the storytelling and characters

Aincrad is by far the worst arc in terms of story and character development (at least of what I've read and barring the Calibur side arc)