I have a lot less experience with some expansions than others, so looking for some insight on this question.
BfA seemed to have very different quests and zones between the two factions. While Legion seemed to have a much more centralized and neutral story between Horde and Alliance because there was more emphasis placed on class content in that expansion. But what about the others, how would you rank the expansions in terms of faction experience while leveling and questing?
Probably BfA since it was two separate continents, one for each faction. The rest have some differences but the questing all took place on a single continent, so there was a significant amount of cross over.
Bfa has a lot of differences, but vanilla eastern kingdom/kalimdor probably has the most 
bc and wrath have a ton of faction specific hubs as well which is definitely worth playing through the starter zones for both sides, and pandaria had amazing faction specific quests chains as well especially in karasang wilds and jade forest
I wouldn’t really know how to rank them per se, but every expansion apart from legion and SL is a questing gem from a faction perspective
Did really enjoy the alliance chain in stormheim though
BFA was unique in that aspect. All the other expansions had a few quests here and there that were unique but for the most part they are variations on the same theme.
WoD might be the next biggest different one since A/H have completely separate starting zones. And unique garrison aesthetics too. The rest of the zones are about the same though.
Ok here is the list.
Burning Crusade - separate quest hubs and many different quests
Wrath of the Lich King - separate quest hubs and many different quests
Cataclysm - many quests that are similar across both separate and shared hubs
Mists of Pandaria - separate quest hubs and many different quests, with some shared
Warlords of Draenor - separate hubs, but many of the same quests
Legion - half and half
Battle for Azeroth - many separate hubs with different quests
Shadowlands - many shared hubs with similar and shared quests
Dragonflight - many shared hubs with similar and shared quests
I hope it helps.
The 4 xpacs with the most separate/different quest hubs and quests are BC, Wrath, MoP and BfA.
Cata Low Level Zones, TBC, WotLK, MoP, WoD, BfA - these have separate quest hubs for each faction in all the zones.
Cata, Legion, Shadowlands, Dragonflight - have some different quests, but the majority are from the same hubs in each zone.
Remember, there are 15000+ quests to complete for each faction with only 25% or less being similar or shared.
And 18000+ quests to complete, if you did everything in Vanilla.
Plus 1000 or so daily repeatable quests. So theres a lot to see and do! Enjoy!