Hi, I leveled a fair bit of alts. On old account like 14 of them, here 3 more. Since shadowlands is getting a leveling revamp, this is a quick-pick tier list of leveling zones. Want to know your opinions as well if possible 
Classic zones, level 1-60: Tier 1
These are very fun to play. Especially zones like Duskwood or Elwynn Forest. Quest flow is great, even without a flying mount it goes very smooth. There are no annoying escort quests, no low drop rate quests etc. You got a rare enemy spawn here or there that grants a good bonus of experience. And there are so many zones to choose from here, you can’t be bored of them if you got 25+ of them to pick from. Dungeon content is top tier as well, some of them are somewhat dated but again, there are so many to choose from that you won’t notice.
TBC zones, levels 60-80: Tier 4
A polar opposite of Classic zones unfortunately. Might have been epic in 2007, but now this is just not fun to play. Lots of annoying quests, quests with extremely low drop chances, huge distances between quest hubs (even a flying mount doesn’t make this tolerable). This is by far the worst area to level in, but is redeemed a bit by some of the best dungeons. I wish i could play those dungeons in Mythic+, they are that good.
Wrath zones, levels 60-80: Tier 4
Also bottom of the list, primarily because you do most of the levels in rather boring zones of Borean Tundra or the other one. Quest chain leads to Borean Tundra though 
I really dislike leveling in this bracket and I’m very thankful to Blizzard for the heavy XP req reduction that happened in patch 7.3.5. These zones are a bit better than Outlands but dungeons don’t stand up to par with TBC ones. In fact, Wrath dungeons are some of the weakest. Good dungeons in Wrath were later patch content, and these aren’t available as leveling dungeons.
Cata zones: levels 80-90: Tier 2
Excellent quest flow and you can do it with 310% flying without any restriction. Decent dungeons too. Would be tier 1 if you could pick any of the 5 zones without any level restriction. Most players just do 80-90 in Hyjal and the other 4 zones aren’t used.
MoP zones, levels 80-90: Tier 3
Now this is a significant step down for me. Jade Forest was a good zone, but the rest is lukewarm at best. Thankfully you can skip the rest now. Dungeons are way inferior to Cata ones, which is why this is getting much lower tier than Cata bracket. Some people do prefer questing in MOP though and I could understand why.
WOD zones, level 90-100: Tier 1
The apex of leveling content, it just doesn’t get better than this. Even without flying these zones are amazing to quest in, you have plenty of bonus objectives, rares, treasures, all of them carry significant EXP rewards. It’s perfect. I would quest in there in Shadowlands for my 1-50. Back when this was retail version i kinda disliked Talador.
Legion zones, levels 100-110: Tier 2
Placing this lower than WOD as there is less exp for bonus objectives and rares/treasures (these often dont yield any exp). Those bonus objectives are also the most hated type of quest - fill a bar quest with activities granting very low %. Invasions help somewhat but they are not always up. Overall good zones but a step lower than WOD
BFA zones, levels 110-120: Tier 3
The reason why i place this on Tier 3 is because of the dumb world quest requirement, that makes it mandatory to have all 3 base reps at friendly. Which means that even if you clear a zone fully and hit 120 with it, you will still be questing (at level 120!) until you unlock the reps. You literally can’t progress your character in any way until you do that. WQs don’t unlock, neck quest doesn’t unlock etc. After BFA these will propably be Tier 2. Nice zones but nothing outstanding. It’s not WOD level of questing. There are escort quests present, and it’s more than one escort quest with NPC leisurely walking for a mile. This is never going to be fun Blizzard. Just don’t make escort quests in your game.
Since flying will be based on your level in Shadowlands i really wonder how they will handle players that choose Cataclysm for their lvling experience. This expansion was designed with flying to be available at all times, which won’t be the case for levelers in there.