Vanilla: Vanilla didn't have an over-arching story, so there's that. Zone based stories were lacking aswell, there might have been some flavor here and there, but most of the quests involved doing random stuff for the sake of exploration. Not much to judge here.
BC: Just a pure mess. They forced in so many WC3 characters with questionable motives that the whole "plot" didn't make much sense. Again, there was a lack of over-arching story, and zone-wide stories didn't make more sense either. I'd say this was even worse than Vanilla.
WotLK: Very good premise that was delivered upon. However, I feel the narrative stumbled in the middle with the Ulduar and Trial Tournament stuff. It had a very promising start with all the zones heavily featuring the LK with Naxx returning, but story-wise we went doing semi-random tasks after that only to walk into Icecrown to kill the bastard. The setting was perfect tho.
Cata: Cata was the first expansion with good over-arching storytelling. By good I mean that things actually followed each other in a logical pattern (more or less) to form a coherent narrative. Now that narrative might not has been the best, but I enjoyed it.
Also with cata you have to mention the revamped old-world. The new 1-60 leveling zones are mostly perfect. They manage to both follow-up on vanilla stories and expand with the cata setting. 5/5 there, just don't mention Winterspring.
MoP: The overall story was good as far as presentation goes, but the choices they made were complete 180 turns, especially regarding Garrosh. Garrosh was established so good in Cataclysm, and then immediately butchered in MoP. It's a shame. The middle part of the expansion were also a mess with the Thunder King, he had no presence, just showed up to die basically.
WoD: Holy god. Multi-dimensional time travel. Why did WoW introduce something like that. I mean I know, the Warcraft movie and all but damn. They could've done literally ANYTHING in this expansion to make it coherent, as the setting didn't force them to stick to any established lore, yet they managed to competely abandon it. Not even do it badly, but abandon it. The intro scenario is the best thing in this expac. Each warlord gets a title card and boom. Some show up somewhere doing something then they're dead and we're back on Azeroth. At least it didn't have any major effect on our main universe.
Legion: Goddamnit it did. Goddamnit Gul'dan. Now, apart from the whole time-travel Gul'dan stuff, it was alright. The Legion went pretty passive after our defeat at the broken shore, at least we had time to deal with the Nightmare and Suramar. The story really picked up after that which was fine. Most of the lore stuff were hidden in order-halls which were just fan-service. They closed some long-forgotten plot-lines/holes so they don't have to deal with them later. Some were fun. But someone could tell us what really went down with Neptulon in Cata - Legion period.
BfA: The story is more coherent so-far and the previous two expansions. It might not be "good" for some, but it's more connected. And I tend to value that more when it comes to storytelling. The Zandalari isles are one big story, it's a joy to play through. Kul Tiras is more drawn out and I feel it has more focus on Jaina than Kul Tiras, which is meh, the fleet could've gotten Jaina's death realm part of the story imo. Overall it's not the worst.
Ranking:
WotLK
Cata
Legion
BfA
MoP
WoD
Vanilla
BC