I voted for
Mists of Pandaria.
MoP not only continued the Horde vs Alliance storyline, but also managed to introduce a brand new setting with never before seen factions and aesthetics and lore to the franchise, and it still felt cool and appropriately Warcraft (unlike Shadowlands where the new factions and lore felt generic and not very appealing like the generic gothic vampires and the two Scourge rehash factions, or out of place for the franchise like tumblr fairies or the passive blue greek angels). The Mogu Empire, the Mantid, the Sha, and the August Celestials were particularly cool. I liked finding the lore scrolls and monuments everywhere, and the Wandering Isle and the Jade Forest were very densely populated with villages and hamlets and NPCs everywhere, making Pandaria feel like a real lived in place that none of the other expansion lands did. The Valley of the Four Winds had a comfy low stakes storyline. Krassarang Wilds was a relaxing place even if the questline there was short. Townlong Steppes and Vale of Eternal Blossoms were pretty beautiful asian fantasy places. The Dread Wastes was a unique final zone.
If I had any complaints, it is that the Pandaren could have used more of an edge. Their ingame models do not look as fierce as Wei Wang's, Bernie Kang's, and Samwise's 2D art, and in the story most of them are portrayed as jovial good-hearted folk, with Taran-Zhu being the only exception. In the concept art, there were going to be four Pandaren clans like a militaristic Tiger clan, but this idea got dropped. Also, it a little disappointing that there was no big capital city.
Fantastic soundtrack by Russell Brower, Neal Acree, Jason Hayes, Edo Guidtotti, and even some Jeremy Soule in there. I liked the voice acting.
Monk class was neat. I liked being able to roll, and the flying serpent kick is cool. I liked the combat healer Fistweaver style, which was most fun to play as and fight against when doing arenas.
I quite liked how the reputations had questlines, so once you finished levelling you were not out of story content to do. I particularly liked the 5.1 Dominance Offensive/Operation Shieldwall questline, where you are researching Pandaren artifacts and also doing sneaky operations in the vanilla Horde/Alliance cities. The legendary questline where you work for the morally dubious Wrathion operating out of a hot springs inn was interesting.
Timeless Isle was the last time I felt like there was an active community ingame, with the zone chat being constantly active and people talking about what's going on in their lives or asking for help to fight a rare.
As for the other expansions:
The Burning Crusade: I liked the high fantasy aesthetics with the holy Draenei and their crystaline structures, the dried up lands of Hellfire, the green lava lands of Shadowmoon Valley, the weird Terrokar forest and Zangramarsh, etc, but the music was too ambient, and there isn't a good singleplayer questline experience like with latter expansions.
Wrath of the Lich King: I remember liking the atmosphere of stepping off of the Zeppelin in the huge, vast Borean Tundra, but otherwise didn't feel the expansion that much. The trailer cinematic hypes up fighting against the dangerous Scourge armies, but ingame you're just wandering around these hubs blowing up easy fodder mobs. The ice zombies thing is a little generic. Storm Peaks had some nice music, but overall I was not enamored with the expansion.
Cataclysm: it is underrated for the revamp of the pre-level 80 zones. A lot of the low level questlines like Gilneas, Silverpine Forest, Azshara, Stonetalon Mountains, etc, are pretty fun, especially the Horde vs Alliance storylines. I also liked how the expansion brought these old zones forward in time so you get a feel for what is going on in your faction years after Vanilla. However, if you were playing a level 80 character looking forward to the brand new stuff, then it is easy to be underwhelmed with boring aesthetics of tree hugger night elves, generic dwarves, generic egypt stuff, generic fire elementals, etc. Deathwing also had a bad voice which made me want to mute my audio whenever he showed up.
Warlords of Draenor: it has gorgeous zones with Shadowmoon Valley, Frostfire Ridge, and Talador. It has a fun questing experience. It is tied with MoP as far as the best expansion soundtrack goes, really amazing stuff there. The evil solar Arrakoa stuff in Spires of Arak was interesting. The expansion was also good if you liked traditional Horde vs Alliance aesthetics, namely Draenei and Orcs, you got a lot of material there. I liked the garrison where you could collect NPCs, plop your buildings down, and finally be acknowledged as a big deal in your faction, being personally appointed by the High King/Warchief to be the leading general for this expac. Unfortunately, the zones often felt claustrophobic and too tightly packed and a little unnatural. There was also a lack of things to do once you had finished the levelling questline. This expac was notable in that sword and board prot warriors could choose to become a DPS role by selecting the Gladiator Stance talent, which gave more player customization. Wish that had not been removed and Blizzard added more talents like that, so that 2H wielding paladins could tank and such.
Legion: a very popular expansion but I was lukewarm on it. Like with the Scourge in Wrath, the marketing hyped up how this was going to be a huge world wide invasion, the War of the Ancients 2.0, and the login screen music was named "Kingdoms will Burn". But rather than seeing Ashenvale burn and Stormwind destroyed and so on, you are instead running around the new expansion island blowing up lots of fodder demons for whatever sidequest. There is little sense of threat outside of the unexpected death of Warbrave Oro, who wasn't even killed by demons but by the new ogre/mole people. Most of the class order hall storylines were mediocre, particularly the Death Knight story which was utterly nonsensical in how you betrayed your Silver Hand brothers by marching into their hall and killing paladins left and right to steal Tirion's body, only for Bolvar to say "just kidding Dairion is the last Horseman" and then this is never brought up again. I was not fond of the aesthetics which was a lot of rehashing of stuff I was not interested in, namely Night Elves, Vrykul, and classical Titany stuff. The neon green lava demon stuff got old fast. Soundtrack was also overall forgettable compared to prior expansions. Lastly, Legion was the start of the world quest format of casually flying and then suddenly a floating head starts monologuing at you.
Battle for Azeroth: once again the marketing got me really excited at the idea of there being this huge world war. I thought it was going to be more of the low level Horde vs Alliance Cata questlines like Stonetalon Mountains, Gilneas, Silverpine Forest, etc. The pre-release War of Thorns questline was fantastic and I wanted more of that. But then the actual expansion launches and you do not get to participate in the big world war. You instead get shipped off to the new expansion islands to go fight generic evils for the locals, like fighting blood trolls, witches, classical Titan stuff, purple tentacles, etc. And then the climax of the expansion was fighting against boring purple tentacles, with the actual ending of the Horde vs Alliance war happening in a cinematic in which the big bad evil Sylvanas flies away, which was unsatisfying. I liked the two new cities. Dazar'Alor with the red brick pyramid and lush jungle and the dinosaurs bathing in the river was visually pleasant, and I liked how huge and immersive Boralus was. I found the moment to moment questing experience to be forgettable. I completed Drustvar, then gave up half way through Vol'dun, and then wound up just doing Island Expeditions until level 120 because that was more entertaining.
Shadowlands: I think this expansion is underrated, though still not very fun. I quite liked the concept of Covenants. The Warcraft franchise is about playing as different visually distinctive factions which fight each other, be it Horde vs Alliance in the first two RTS games, or Night Elves vs Scourge vs Legion in TFT. Shadowlands introduced four new factions you could join with your preexisting character you are already invested in without having to leave that behind and reroll a new character. Unfortunately, the four factions were unappealing (passive blue greek angels, watered down Scourge rehash, tumblr fairies, generic gothic vampires), and they did not fight each other. I liked the fantastical environments, with the floating golden islands of Bastion with the blue floating strands of anima overhead, Maldraxxus where the land is made of flesh, Ardenweald with the huge trees with stars in the canopies, Revendreth with the blood rain, the hyperspace flightpaths, etc. Shadowlands had sadly the most utterly forgettable soundtrack in WoW, with no good music coming until Zereth Mortis in 9.2.
Dragonflight: I liked the city of Valdrakken with the huge stone tower and the verdant green grass and orange/yellow and purple plants, and the Waking Shores zone with the brown canyons and water flowing through them. The Emerald Dream zone also looked gorgeous. Music was slightly better than Shadowlands. But once again a lot of the aesthetics are rehashes, with more generic dragons, generic Mongol centaurs, more Tuskarr, fire Vrykul, more elementals, etc. The story was also very bad, actually the story had been bad since Legion/BFA but here you have lots of cutscenes and quests where characters stand around and talk, and I quite dislike a lot of these people.
The War Within: I always wanted an underground expansion and was excited when it was announced, but you only get three underground zones, which is really frustrating because there was more than enough material to have 6+. The factory area of the Ringing Deeps could have been a zone. Easily could have had a zone where you trawl through the guts of some huge organism like an old god, like with the Black Blood area of Azj-Kahet. Hallowfall could have been bigger, like the size implied in the second CGI cinematic. The Isle of Dorn surface zone is pretty forgettable, and I did not like the dwarf/classical Titan aesthetics of Dornogal. Dornogal is pretty underwhelming compared to Valdrakken and Oribos. Also, the title and the key art for the expansion advertised that you would get a big war experience, but then ingame you're just running around blowing up a few mobs, and the big battle advertised on the loading screen only happens for 2 minutes as you fly around burning down three mobs. Very disappointing.
