I didn’t play WoTLK, Cataclysm, WoD.
I can’t speak for those expansions.
I would go for Vanilla, even though that’s not a expansion.
I guess nothing beats your first time in the game.
We had Rend Blackhand, Ragnaros, Nefarian, Onyxia true villains to beat.
For expansion, I would chose TBC.
Thrall was still in charge.
I still played with my RL friend and had a Guild.
Kharazan was the first raid I managed to clear, with the rookie Raiding team of my guild.
The downside, was: it started to devaluate Epic gear, introducing dailies with isle of Isle of Quel’Danas and putting characters as Raid bosses, for the sake of raiding.
Illidan, Lady Vash, Kael’thas, Zul’jin.
That’s why I probably enjoyed Legion so much, proper villains instead of “Morally grey” characters.
I think Wrath of the Lich King was the first expansion that made my jaw drop, fantastic lore, packed great accessible content, huge areas to explore, fantastic quests and even great Hunter pets and things that you had to spend a bit more time getting hold of but made it all the more exciting and satisfying when you obtained them.
Given that I haven’t experienced any expansions from prepatch to prepatch but rather quested through them all during BfA, I can’t tell how my opinion would have been if I had that experience.
Although I was never there when the content and patches got released, I have during my loremaster-questing tried to start where I would had started in prepatch/initial patch, and moved along with the storylines and available content without being affected by content halt/lack of content.
So, after giving it some thought, here is my ranking:
TBC = WotLK = Cata = MoP = WoD (all equally different with great questing and story experience)
Legion = BfA ( BfA have better initial story questing experience through the new zones, but the daily chores for potential gear upgrade makes it far worse than Legion.)
I loved Cataclysm. I have strong memories of revisiting each zone in the world and seeing what changed, reading the quests, the emotions they brought out of me for the first time of seeing NPCs I just spoke with dead. The Forsaken areas made me -really- hate everyone else.
The update in graphics was awesome, Stormwind especially, it was so shiny and sharp! I loved most of the new zones my favourites being Hyjal as a Nelf Druid and then Uldum for obvious theme reasons! It all flowed so well together. This was the first expansion I played and levelled properly rather than just socially messing about with friends. I have great memories of it, especially because I was maining Druid.
I have enjoyed all the expansions even this last one. I think I never want to see murlocs or naga ever again. The only part I really did not like in Legion was Argos . Pandaria will visually always be my favourite
WotlK -no competition. After this expansion it went downhill, moving away from it RPG roots. MoP and WoD I didn’t play because of this but decided to return for Legion. I thought the content was great in Legion but hated classes, the concept of borrowed power which has no place in a classic RPG and neither does the insane game systems that the dream up every expansion.
Shadowlands is the last try for me. If the next expansion follow the same rout I’m out again.