I mean, mine are TBC and Legion for S teir. Then I do a massive hot take which i imagine nobody will agree with.
LFG and Flex difficulties were the worst things they introduced. They ruined the sense of community and progression, in TBC I knew a ton of the ppl on my server and had a friend’s list of ppl who I knew were solid for HCs. Now I don’t know or care who the M+ members are.
A tier: WotLk, BC, Legion
B tier: MoP
C tier: Cata, WoD
Crap tier: BFA and shadowlands.
A tier: WOLTK SL Legion
B tier: BFA mop Cata
C tier: TBC classic
S:- TBC, Legion
A :-Classic, WOD, SL ( so far so good )
B :- BFA (only played BFA in the last patch) WOTLK
C :- MOP, CATA
S tier: everything until wotlk
B tier: bfa
C tier: legion and shadowlands
D tier: wod
F tier: cata
skipped mop.
Garrison was a huge part of its content you was forced to do it… Oh yeah tanaan jungle was really nice and shipyard, so good. 
How was it forced? Shipyard goes under garrison. Mission table had good rewards but was not needed, only problem with tanaan was that the patch lasted to long. But it was a great way to catch up your alts.
Best: wotlk, legion
Good: tbc, mop
Avg: cataclysm
Horrible: sl, wod, bfa
S tier = Vanilla (mainly cause all where noobs, both regarding the game and the games social structure)
A tier = Legion (Artifacts, still miss them), MoP (Monks, Throne of Thunder, awesome zones), wrath (Arthas+Ulduar+Howling fjord and grizzly hills music)
B tier = Cata (Kinda boring), Bfa (Azerite gear was the worst), BC (bad lore but still Karazhan)
C tier = Wod (hate garrisons so much), Shadowlands (boring theme, gearing sucks)
S: Legion great Story, Dungeons ans Raids
A: BFA Zuldazar was an awesome city with flying. Zandalari Trolls <3
B: SL Too much timegated. No loot. Boreghast, Some Dungeons more like a boss rush
I started in Legion, so far I prefer Bfa over both legion and sl. Castle Nathria is the biggest plus of Sl.
S: Shadowlands , legion .
Shadowlands S tier
all other expansions are under it, call it A or B except BfA, BfA sucks, BfA is trash, BfA is the worst thing that happened to this game, not WoD.
I only got two tiers.
Had fun: BC, Wotlk, Cata, WoD, Legion
Made me quit: MoP, BFA, SL
My tiers:
Really enjoying what i’m doing: TBC, WoTLK, SL so far
Enjoying it more than i’m not: Vanilla, Cata
Questioning why I still play at times: Legion
Made me take a break: BFA
I skipped WoD and MoP due to RL stuff.
BFA is the only expo in WoW’s history that caused me to actually disengage from the game, unsub and not play for a reasonable chunk of the expo. That expo felt too much like a goddang job with all the crap it made you do, so when “let’s now run visions every single week as well! and more world quests in these zones! Oh and more essences to grind!” I just noped out. Came back just before pre-patch hit.
If the last patch had drastically changed the way things worked i may have stuck it out but by that point all the fatigue of grinding across main and alts, to be confronted with something else that was going to take yonks before I can enjoy it properly was just a huge slap in the face.
S tier: Wrath, SL
A tier: BC (resil hitting crit builds was sad), Legion (gutted sub), Mists (great mechanically but meh story)
B tier: Cata
C tier: BFA (ergh)
F tier: WoD (didnt play it, alternate timeline did nothing for me)
S: MoP
A: Legion, SL, classic
B: Cata
C: WoD
Z: BfA
The others I can’t comment on as I didn’t actively play, but probably TBC in S tier and Wotlk in A tier from my ehem experiences of them.
Ppl who put SL in S tier should visit some doctors! They should check their mental health.
S tier: Legion, MoP
A tier: BFA, WoD, WoTLK,
B tier: Classic, Cataclysm, Shadowlands (?)
C tier: Burning Crusade (Worst zones, terrible lore making Kael, Vash and Illidan such bad guys just to be bosses.)
S Tier: MoP
A Tier: Legion, WOTLK
B Tier: TBC, Vanilla, Cataclysm, Shadowlands
C Tier: WoD
D Tier: BFA
I can’t judge SL fully yet cause it still feels bare bones in content and the catch up systems rely to much on RNG for my taste. Also blizzard messing with loot at the start left a sour taste in my mouth.
If the loot changes never happened and the renown catch up was more consistent or explained i’d bump it up to A tier or possibly S Tier.