The only time I’ve ever really paid attention to the lore was when Sylvannas became one of the focal points; so I would like to say hers—but we all saw how that ended, so yeah, I don’t really have a favorite storyline.
The death of Sylvanas at the wall at the hand of Godfrey. The story gives you an insight into the life of Sylvanas especially when she tells you of her demise at the hand of Arthas and then being raised as a Banshee. She tells you all of this as you ride by her side.
If we’re going by expansion, probably Legion. If not and you just mean anything in general, I’ll choose the Zandalar Forever storyline (Seeing as Silverpine was taken). So freaking epic.
I’m gonna go with a few good ones from classic wow:
All the stories tied to Onyxia’s intrigues in Stormwind, which means: The Defias Brotherhood questline starting in Northshire Valley and ending with the Lescovar Incident, The missing Diplomat questline and the Great Masqurade (Onyxia attunement). The multilayered intrigue was really great here.
The Scepter of the shifting sands questline
Fallen hero of the horde questline
One thing that’s common for all three is that they happen in multiple zones, sending You to places that You’d not otherwise visit. It really made the world seem real. Today, if the quest requires rare herbs/ore/other itmes or consultation with someone with unique knowledge, somehow very conveniently all this things are just a few yards away, in the same zone. It’s very immersion breaking for me.
Honorable mention: Rhok’delar, Longbow of the Ancient Keepers questline. Not for the story, but rather due to the quest design and skill requirement involved.
In terms of general side stories… quite a few honestly. I tend to like those more than the main plots. I think out of those the dwarf one in dragon flight is my fav at the moment, it’s a goood bit of world building… that will never come up again in anyway but in isolation it stands well.
For expansion plots… all and none. They all start strong then drop off so hard it feels deliberately bad.
For all the issues that WoD had, and it had a few, the storylines that WERE there were great and the art team knocked it out of the park. Biggest issue I have with WoD storyline is that it feels fragmented because of the cut content, so some parts of the story make absolutely no sense (Like Yrel’s relationship with Grommash, this skipped a major planned part of the story).
Personally I enjoy Wrath storyline, because the overarching Bad Guy of the expansion was in every single zone in some capacity, so it made the story seem more wholistic.