Vanilla
The Burning Crusade
Wrath of the Failking
Catalol
Warlols of Dreinlol
Pandorilol
Lolegion
Crapsmcgee
As much of a hot topic is for Vanilla and how much I liked it, I didn’t really love and invest my time into the game until BC.
BC showed a lot more balance in endgame (and a lot more imbalance) and was just a lot more fun for me.
Disclaimer: I only played from mid Vanilla until the end of Cata so most of the list is unknown to me.
I think Vanilla was probably the best overall game, TBC had the best endgame, and Ulduar was my favorite raid. I had some fun in Cata arenas and I enjoyed Firelands but Cata is still easily the worst version of the game that I’ve played.
Prob enjoyed the content of WotLk the most… prob had my most fun while mythic raiding in WoD, would love to get back into mythic raiding but hard to get back into.
Always burning crusade for me. I loved the state of the game in TBC.
New world to explore, arena, new BG, far better class balance, awesome itemization and amazing endgame that had multiple tiers available at launch that had to be progressed through and unlocked, instead of being time gated by patches.
BT, ZG and the war bear timed run were awesome and then you have sunwell isle with cool new dailies, a new dungeon and a raid only the most dedicated could reach.
Crafted gear was useful and some lasted until ssc/tk/hyjal not to mention new patterns in those raids and beyond to SWP. Also flying added a new dimension to gameplay in the world, I firmly believe flying wasn’t an issue until blizzard decided it was an issue.
I remember crashing when the isles came out. 
running heroic MT and replaces t5 equipment.
TBC>Vanilla>WotLK probably, i enjoyed myself the most in TBC, but played and achieved the most in WotLK, but then again so did everyone as WotLK was such a welfare expansion… Everyone was pugging 25M ICC at the end.
As an overall game Vanilla is a clear winner for me.
TBC wins when it comes to raid and dungeon content.
Other expansions are better left unmentioned.
I enjoyed WoW while the community aspect of the game was in top form. Back when you knew all the heros and villians of your realm. Somewhere at the end of TBC, it seemed like the folks I randomly interacted with started becoming faceless names, communication became less and less, and once this happened I just lost my connection to the world/game. When I played WoTLK, besides saying hello to folks on my friends list, the game felt single-player to me.
Mists of Pandaria had some of the best pve and pvp content ever placed into the game. A lot of top notch world building with a strong core narrative. It provided ample RP opportunities and was just fun to experience.
TBC only because season 2 mace stun on my warrior with stormherald was way too fun.
TBC was the best WoW ever was… it improved Vanilla without sacrificing what made it such a great game.
Vanilla was probably the most balanced out of all of them. Especially near the end.
What Vanilla did you play?
What Vanilla did you? Every class was pretty strong and had a role to play.
We talking PvP or PvE as what I remember there were huge gaps between gear and classes for both.
Vanilla and Burning Crusade for me. The game started to decline for me in Wrath, even though it has my all time favorite raid and a great story/zones. From Cataclysm forward, couldn’t care less about it.
I enjoyed, the pace of Vanilla, the raids of BC and WOTLK, (Boooo flying mounts), the Arena, the World pvp of Vanilla…
I want Vanilla world, with some BC and WOTLK raids and an optional Arena. NO FLYING
I started in TBC, but didn’t really get to end-game. I liked WOTLK the most, at the time at least. Legion was fun as well.
-Cata was interesting at the start, overall ‘ok.’
-Mists had interesting PVP, but overall I disliked the expansion. I’m probably the only one who liked doing scenarios with friends/guildies lol.
-I was unsubbed for most of WOD. Came by at the end and enjoyed it while I could, but overall there wasn’t really much to do that was exciting, but considering how late I came back, I didn’t get completely worn out yet.
-For BfA, there is a lot to do, but I just don’t want to do it. Nothing really makes me want to play. I did flying and had a lot of fun at first doing the zones, but they wore me down really fast. I haven’t even bothered to level another character to max yet this expansion.
My personal enjoyment of the game:
I think TBC was actually a better era than vanilla or WotLK, but I personally didn’t have as much fun then, just because of the people I played with and guild situation.
The only real outlier to my recollection was Hunter. They didn’t scale as well as most others. They still had their role to play though. Everything else was close enough.