This probably isn’t going to be an opinion many share, but Legion Arcane Mage.
The spec went from S tier, Primsatic Crystal + PoF + Arcane Missiles pumping into top DPS in Hellfire Citadel, to Arcane Explosion spam and Mark of Aluneth burst.
Granted this is heightened by Hellfire Citadel being peak Arcane Mage and then Legion moving my focus from Raiding to M+.
But still, I exclusively played Mage from Cata through to Legion and Legion was the expansion that ultimately made me change.