I’m going to post this by order of release.
Vanilla - High C: This is the beginning of WoW, and while it doesn’t follow the story structure that’s been set as a precedent for most of the expansions, that’s fine. It wasn’t about any particular characters, it was about the world itself. It’s a style unto itself, and it works when playing the game. Though in regards to storytelling, it’s not that great. Vanilla is about the environment and experience, not so much the story.
TBC - Low C: This expansion builds slightly upon how story is told in Vanilla, making it somewhat clearer, though still vague in other parts. Lore characters begin to get more focus here, for better or worse. It feels like Vanilla, with more special story moments put in. While this is nice, some characters could’ve been dealt with more respectfully. It lands at a strange spot, of not being good but not bad either.
WotLK - High B: This expansion is when the story gets cinematic, giving it focus, with clear explanation and exciting cutscenes. Lore characters become main characters, and the ones in this expansion are likable. Every moment keeps your attention, and none of it is too complicated to understand. While this might be most people’s favorite, I have a personal bias against undead. WotLK is no doubt great in all aspects, but my bias is what prevents me from rating it an A.
Cataclysm - Low B: This expansion starts with a bang, as we see when Deathwing breaks free, giving us another worthy villain. We’re given reason to visit the original zones again, with a focus on the elements. Not all of those zones have the same tone though, giving players tonal whiplash. Thrall is also the main character, who is just a self insert of Metzen this expansion. The overall story is good, but it has missteps.
MoP - High A: A story about the faction war, it starts without a world threat, and when antagonists become known they’re great. Pandaria gives themes which weren’t dealt with before, through incentive which every player at the time could care about. Since there’s no antagonist at the start there is a feel in the zones that’s similar to vanilla, and everything is a new concept. The lore characters involved are interesting here, including the villains when it becomes known who and what they are. I adore MoP, an expansion that gets the story right on nearly every level.
WoD - F: A story with many flaws, involving a weak threat, from characters that have already been dealt with. Draenor is a great setting, but how we get there nearly destroys established lore. The Iron Horde is little more than an annoying gnat, with cool looking technology. This might be WoW’s only filler expansion, with characters that aren’t bad but are meaningless. This stepping stone of an expansion ends on a loud fart, which sounds like someone saying “Draenor is free”.
Legion - Low A: It’s a story that ties up several loose ends, involving a threat which players have been anticipating, along with an enjoyable cast of characters. We go to the Broken Isles which has a diverse set of zones, building upon a setting which has been known about but unseen for years. The Burning Legion causes damage from the start, and we take bigger steps at each part of the expansion to go against them. This is what WoW has been building up to, and outside of faction conflict it’s nearly perfect. It’s not rated higher than MoP because the Burning Legion is defeated more easily than expected, and the Horde isn’t shown enough.
BFA - Low D: This expansion has two stories, which is the faction war and N’zoth, giving us a mixed bag of characters. Kul Tiras and Zandalar are wonderful, which are exciting places. But there is no denying that the faction war here is a rehash of MoP, which ends on a weak conclusion. The story with N’zoth is better, but could have easily been more satisfying. This is a disappointing expansion, because without the faction war it could’ve easily been great.
Shadowlands - High D: While the expansion isn’t finished, we have played through the setting that’s involved, and deal with the bland characters from BFA. The zones are interesting, but unexpected in a way that isn’t entirely positive. Villain motivations are vague here, yet ironically simplistic. Zovaal is plain, and has been described as a discount Thanos. This is what we know, and what happens in the future of this expansion probably won’t change how we feel about what’s already been described.