Not particularly loyal to any of my chars, although do mostly come back to warrior.
Vanilla: Shaman
Vanilla Oceanic Server release (about a year after original launch iirc?): Ally Warrior
TBC: Warrior, then horde mage + hunter with RaF boosting.
WOTLK: Ally rogue, then back to my Warrior.
Cataclysm: Warrior and mage, but quit 5 months into expansion.
MoP: No real main for the whole time, played tons of classes, Warrior, rogue, priest, hunter, shaman, lock, mage mostly
WoD: Mained ally druid, ally hunter#2 (locked xp on my old hunter), ally lock and horde priest mostly, did a lot of alt-ing getting all classes to level cap.
Legion : original Warrior only, legendary system stopped me alting, quit a year out from BFA.
BFA: warrior, then a new hm druid and hm hunter. Quit a couple of months out from classic.
Classic: Ally rogue.
Shadowlands: will probs go warrior, though not sure I'm invested enough to play retail anymore. I'll give it a chance, but my expectations are low.