Personnaly I find a few different answers to be all as valid as one another.
If we're talking about playing the xpac as a max level, I only did that during DF but I sure as hell had a blast. Beautiful, huge zones, pretty cool factions on the Dragon Isles, nice lore on side quests (not as good for the main story, especially the last patch sadly), even if the story was meh for the patch, access to the Emerald Dream is very enjoyable.
The loot system was simple enough for a casual like me, dungeons were accessible, Resto Shaman felt great (at my level ofc) and dicovering the content felt very refreshing since I only ever played trial before.
If we take into consideration Classic then that would take the cake but I was barely even alive when the game came out so no real Vanilla for me.
That said Classic was still a mighty fine adventure, nice people, slower gameplay, good immersion and a "fresh" vision of the old world for me. Class fantasy is on point and even if I suck, I enjoy me some Classic from time to time.
But realistically the expac I had the most fun with was SL, purely trial, the whole world was fresh and asking to be explored (well besides the few zones we had available before which I still enjoy very much). The power level felt -at least for me- a lot more pleasant, ground mounts only would be kind of a pain now that we tasted flying but I believe this restriction added to the excitement of exploration. Only knock on the xpac at 20 is that most classes felt incomplete.
Now if we're talking about which expansion is the most fun/enjoyable/liked in regards to "how does it feel for us 20s nowadays" and personnal opinions on aesthetics/lore/music, there's a lot of good xpacs like Wrath, WoD (for trials I find the lore pretty solid since we mostly get the war on the Iron Horde and some Shadow Council stuff) and the Vanilla/Cata content. All have generally useful profs (besides maybe WoD but then again it used to be our source of bags), I love the aesthetics and lore, the quests are pretty neat (maybe a bit less for the Vani/Cata zones as they tend to be a bit too streamlined with a lot of zones lacking a real story : then again, it does help feel like an adventurer).
But fuck me, MoP is goated. Content is cool, quests are solid, not crazy, but the aesthetics man. The lore of the continent is surprisingly well-developped and accessible to 20s, pandas are neat, I like 'em, Valley of the Four Winds, Halfhill, having our own little farm, the Anglers are neat, the Grummles are superb, Hozens rock, Garrosh is batshit insane but I don't give a shit I've got a farm and he can't have it.
The Panda starting zone is also fairly good, kinda like the xpac, surprisngly so.
And don't get me started on immersion and RP. The continent is amazing for that.
Only knock would be that we can't have animals on our farm but alas, can't have everything can we ? Stardew Valley exists after all and we can't surpass perfection.
Sorry for the long answer, just a lot of interesting possibilities to approach the question and a lot of good qualities on a lot of xpacs.