Very curious as to what people opinions are. I don't think Warcraft has the best storytelling period, but they're good at making awesome cutscenes for shockvalue or sheer awesomeness. They're even more limited in the scope of an expansion as they have deadlines to meet thus content gets cut and they adjust the story around it. Many expansion story lines start off strong but tend to end on a whimper because of this.
For me it's a contest between WoTLK and MoP so here me out and I'd like to hear other opinions.
Vanilla
Adding this for those who believe a good story in an MMORPG is no overraching story, sure things are going on, but tackling threats is day to day for an an adventurer. Nostolgia for Vanilla is kind of what makes me more open to expansions like MoP and BFA where the theme isn't some super threat trying to kill you throughout the entire expansions life cylce.
The Burning Crusade
More like the Burning Expedition....I love TBC but the story is a mess and before Blizz was even trying to tell a narrative. We arrive in Hellfire where the threat is clear, giant pitlords, fel orcs, Hellfire Citadel, etc. Then we end up in Zangermarsh....and some chick is stealing water. There isn't a strong Legion or Illidari presence until you get to Netherstorm and Shadowmoon. Hell there wasn't even a solid explanation as to why we killed Illidan and the threat of the Legion isn't felt until the Sunwell patch. So TBC feels more like an expedition (that I enjoyed) where you learn more about this broken world.
Wrath of the Lich King
Clear front runner, closing up the story of the fallen prince Arthas, deviating to Ulduar and ending in Icecrown. We're introduced to plenty of new heroes such as Garrosh and Varian building up drama between the two factions. The Argent Tournament does feels out of place, would have rathered the Spider Kingdom or a Gundrak raid. And while we never saw Illidian out side of Black Temple in TBC, Arthas popping up every hour like a Saturday morning cartoon villian shaking his fist at you is kind of comical in hindsight. Malygos jumping off the deep end also makes no sense, I can understand he doesn't want mortals reckless using magic, but going to war with the Kirin Tor when the Scourge is clearly a bigger threat?
Cataclysm
Cata was a mess. After taking down the Lich King the biggest villian we've vanquished an even bigger moar massive villian pops up out of the earth breaking it. Great for promotional art, but as far as story goes? Deathwings personality leaves so much to be desired, he's just a crazy dragon, and how he amassed such a force of followers so quickly and randomly is also confusing unless you dig into lore. It's a weird amalgamation of Elementals, Dragons, Twilight Cultist and Naga, all tied together by the Old Gods. Both Nefarian and Onyxia were far more interesting, Nef plotting non stop in his lair, with Onyxia working behind the scenes politically. Then we have Thrall....assigning Garrosh as Warchief (cause what could go wrong?), Green Jesus and the Dragon Aspects losing their power seemed unnecessary.
Mist of Pandaria
I feel like MoP deserves more points for originality and gets a bad wrap for it's panda theme. Though not the most epic I feel like the pacing of the story up to it's conclusion just works. Starting off when the conflict between the Horde and Alliance escalating, a quick respite to deal with the Thunder King, to finally the the two factions uniting to stop Garrosh where we canonically raid a capital city for the first time. Bringing along with it many interesting factions such as the Shado-Pan, Klaxxi Paragons, the Sha and the Celestial Guardians. As always though there are some character developments and choices that were just silly. I mean Varian should have really just let Thrall smash Garrosh's head in (Varian would still be alive then). Take him to trial? The guy is guilty ffs, you wanted to send him to the Phantom Zone or something?
Garrosh going full meglomaniacal in such a way just contradicts his character development from Cataclysm. Rhonin dies in a lame way. And I've always been bothered by Jaina's transformation and views against the Horde, while understandable. Once again an expansion where there isn't an ever present super threat that's going to kill us all is refreshing, and MoP shows that Blizzard can tell new stories that aren't directly tied to WC3.
Loses points for time travel/multi-verse theory off the jump. While the idea of picking off each Warlord one by one could have been great, it was poorly executed even at the start. Kargath dies as the first boss in the first raid, Ner'zhul is a 5 man boss, he's the original Lich King so much more could have been done with him. Gul'dan, Chogall and Teron were interesting. I adored Ga'nar (Garrosh minus the stupid). But this one ends on a whimper Grom is no longer bad and we ban together to stop Archimonde and Gul'dan. Grom isn't held accountable at all. I can barely touch on the lore here because I barely left my garrison.
Warlords of Draenor
Legion
This expanions starts off better than all of the expansions in my opinion. The invasion of the Tomb of Sargeras, death of heroes Varian, Vol'jin, Tirian. Seeing just about every major demon you've ever killed come back to life on that beach makes you want to crap your pants.
Then we have Surumar and the conflict with Xavius resulting in Ysera's death. Azshara re-appears. All of the first half cultimating in the death of Gul'dan a top the Nighthold (similar setting to the Lich King ontop of Icecrown) and it feels like such sweet vengeance for the death of your faction leader.
After that though....I mean we finally get to see Kil'jaeden, but not nearly enough. As far as we all knew this was the effective leader of Burning Legion and we barely get to know him. Less in game developement than Gul'dan...whose from another timeline. I also found the Argus and Antorius plot lines hard to follow being so "alien".
The ending cutscene Sargeras pops out in all his glory from a cloud and I'm thinking..."where the hell did he come from? And where has he been? I thought he was stuck in the nether?" Then Illidan becoming his jailer? How exactly? Sargeras is too powerful to even be a raid boss, and Illidan couldn't even 1v1 Kil'jaeden. Illidian's development was odd to me also, many of us wanted a redemption story, but that's because Illidan wasn't supposed to be evil thus killed in TBC. But everyones favorite demon hunter went from being a self absorbed anti-hero that does the right things for the wrong reasons to someone that is now benevolent enough to spend all of enternity watching Sargeras sit in a chair?
Battle for AzerothSo much potential lost here. The narrative of the Alliance and Horde at full war (and no pandas) sounded great. Saurfang's honor vs Sylvanas's ambition was fun to watch. The problem all lies with Sylvana's and her followers motivations. I remember the fan theories of what set this war off, my favorite being the death of Vereesa Windrunner. If Sylvana's kills her own sister it had to be for something major, or perhaps Vereesa's death indirectly by the Alliance's hands triggers Sylvana's to set Darnassus ablaze, but no Sylvanas is pretty much just being a bitch. And we've seen this story before with Garrosh, yet somehow so many Horde leaders just line up behind her. It's so clear that Lor'themar was wary of her in MoP, yet he just falls in line? The Forsaken were always outcast of the Horde, never trusted. Sylvanas being promoted to Warchief during the Legion invasion could be rationalized by her experience and being the longest reigning faction leader. Invasion is over, time to remember this chick was always quesitonable. I was interested to know what she was up to in Legion, but the Shadowlands set up could have been done without her starting a genocidal war.
I found Azshara charming and glad we truly got to know her, but I feel she arrived waay to late. Cataclysm through Legion was her time. It's amazing how smug she is despite Kil'jaden, Archimonde, Sargeras, Arthas and Deathwing being defeated, girl you aint that special. N'zoth can at least attack our minds, but recent Old Gods to lackeys. Aman'thul literally plucked out Y'shaarj.