For Alliance it's Lok'Modan, Redridge, Darkshore, Felwood, Stranglethorn (both parts), Plaguelands. Somewhere between 3 - 3.5 of these get you to 60 in like a 16 hours /played, if you queue for dungeons at the same time. I recommend dropping everything at 45 and going to Darkshore-Felwood if you didn't do them yet - they are literally the fastest levelling zones in the game right now, maybe only trumped by Silverpine.
Still, it’s terrible to see the new expansion fail so spectacularly at iterating on Legion’s success. It’s hard to believe the two expansions came from the same company. Legion made you feel powerful and unique. Battle for Azeroth grinds at you by taking away as much as it gives. Sure, you can have a shiny toy. Sometimes. If you don’t get too carried away.
And, of course, there are plenty of in-game addons you can use to make your adventures in Azeroth even more efficient. Try checking out Azeroth Auto Pilot, which points you where to go, handles all your NPC discussions and quest turn-ins, and automatically sells your junk gear (among other features). Deadly Boss Mods is still a must-have, so you don’t die in dungeons quite so quickly. Pawn helps you tell whether the gear you’re getting is actually an upgrade or not. World Quests List gives you a much better way to see what’s happening in your zone (and sort for the rewards you’re prioritizing).
Do quests and complete the story of your first chosen area before returning to your capital to scrap all of the equipment items you acquired. You should be around Level 113 by the time you complete your first area. Before selecting your second zone, head to your faction hub ship to acquire your introductory quests to the mission table and your champions. Complete this introduction and then pick up the quest to establish your first foothold in enemy territory.
Get your item level as high as possible. Having the best gear is always going to help you when clearing content, so upgrading your individual pieces of armor will be important in the first few days after the expansion’s release. If you’re looking to get ready for Uldir, then you’ll want to be around item level 350 by the time that raid is released on Sept. 4.
Unfortunately, the BfA prepatch took away more than usual--having an entire expansion balanced around an ultra-powerful weapon that abruptly turned into a dead stat stick made the end of Legion particularly depressing--and offered very little in return. The patch didn't flop because of bugs; it failed because it's boring, unrewarding, limited, unambitious, and actively feels like it's punishing players at a time they should be celebrating.
I highly recommend actually staying in the 20-60 range zones until you ding 62. The reason for this is that despite getting a slight penalty in experience for being over-level, the advantage of having flight will more than offset this. That, and WOTLK and TBC zones are the slowest part of the leveling process. Minimizing your time there as much as possible is worthwhile.
Some people have made arguments that quest blues can sometimes be more powerful than heirlooms, especially with sockets. I'm not convinced. The simple fact that you never have to even look at quest rewards will save a non-insignificant amount of time over the course of leveling. Also of consideration is the value of enchanted gear never having to be re-enchanted.
Azeroth paid a terrible price to end the apocalyptic march of the Legion’s crusade—but even as the world’s wounds are tended, it is the shattered trust between the Alliance and Horde that may prove hardest to mend. As this age-old conflict reignites, gather your allies and champion your faction’s cause—for Azeroth’s future will be forged in the fires of war.
I do not recommend going for TBC, although I remember Hellfire peninsula to be decent, everything else is rather slow. Instead go for WotLK, Borean Thundra gets you levelled very fast, and then you can head straight for Icecrown at 67, which is also insanely fast, although admittedly has a few elite quests. You can go to Sholazar Basin alternatively. 2 zones should get you 60-80 no problem.