I started playing during Wrath but for me, it’s Legion. I grinded to unlock the Nightborne as well as Void Elf and Lightforged Draenei. Enjoyed those quests. Liked the class hall and mount campaigns. Mostly enjoyed the mage tower, raids, and dungeons. Suramar and Nighthold remain possibly the most visually appealing in the entire game.
Legion is my vote. Artifacts were cool as heck
BfA and Shadowlands.
Gonna be honest my favorite expansion (based on the classes I played and how fun they were) was Cataclysm. I really enjoyed Disc priest back then 
MoP and my hot take is that if literally everything else remained the same but the new race was something other than goofy pandas then most fuddy duddys would admit that is easily in the top 3.
Legion by a mile. Wrath was nice, but Legion felt like it had so much more content and had something for everybody.
You had your Mythic +, Pet Battles (I think they came out in Legion?), easy raids (such as Emerald Nightmare) , medium difficulty such as Antorus, and very challenging such as Tomb.
Not to mention, class design was excellent, with PvP actually being fun again. Plus, the class halls were nice and (it might just be me) the visuals and sounds of the zones were fantastic, especially Val’sharah and Highmountain.
It was just an overall very pleasing expansion. I enjoyed Wrath as well, but did not enjoy most of the leveling zones, such as Icecrown and Borean Tundra. Zul’drak, albeit had a beautiful theme (sound wise), the leveling put me to sleep.
MoP was the last good xpac.
I’ll jump on the bandwagon and say wrath. Mainly because of nostalgia and fond memories. There was actually a lot I disliked about wrath. If I had to say a second it would be Legion.
cata & mists were when i had the most fun personally 
the only ones that burned me out the most were TBC and WOD
I want to say Wrath but I also know that’s mostly nostalgia talking, objectively I’m going to say MoP.
I have to say MoP for me. Thats when PVP was the most enjoyable for me where everyone had something that was pretty strong (bring back symbiosis for my resto druid please!), and we actually had to make plays to win arenas. They also lasted longer than 20 seconds which was fun.
Raiding was also super fun that xpac. I can say i enjoyed every tier that was out then.
Depends on what you were looking at.
In terms of coherent story? Wrath, outside of Legion and MoP all the expansions were just nonsensical or a bunch of stories thrown together.
In terms of dungeons and raiding I would say MoP. Outside of Mogushan Vaults (who makes a raid that is 90% tank mechanics) I had the most fun on average with those raids and not so long dungeons.
Leveling? Legion.
Sheer amount of content? Was Cata, people might be thinking I am insane but Cata was not just 80 to 85 leveling zones. It was two new races, and every Classic zone remade with new quests and every classic dungeon remade with new quests and designs. Plus transmog, Have group will travel, guild leveling, and reforging came in Cata.