I've recently did entire loremaster from vanilla -> MoP and the worst leveling experience was uironically... Wotlk. Disjointed, slow, not making sense, not fun and had the most bugs. Second after that was BC, definitely not fun and plain boring. Then there was a visible gap and MoP/Cata, which was overall pretty mediocre but had some good moments, then WoD which was just super streamlined and not very interesting. Then legion (first modern experience that was somewhat entertaining), bfa was around the same level.
Can't speak for SL as I have yet to finish it. But quite honestly in my book it will be a bit worse than BfA/Legion.
Now the disclaimer:
I did ALL the quests in a timespan of 1 month in order: Vanilla, TBC, Wotlk, Cata, Mop on one character.
And since WoD i am leveling 11/12 characters each expansion.
So overall score would be like:
Wotlk -> Tbc -> ... gap -> Vanilla (maybe due to cata reworks?) -> MoP/Cata -> WoD -> ...gap (modern wow stories) -> SL (probably) -> Legion/BfA
On beta i got stuck 3 times during leveling so I never had chance to finish it.