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.
This expansion suffers from the Warlords of Draenor (WoD) effect, initially, players will think this is a great expansion but upon investing some time into the game they'll realize this game is a disaster. The pre-patch should have been our first warning with how poorly designed and executed that fiasco was, but beyond that, let's look at what the expansion itself offers:
These are a genuinely challenging and dynamic way to play. No two are alike, and the A.I. actually does give you some real opposition. Island expeditions are like dungeons, available in normal, heroic, and mythic difficulties. That means you can press into higher difficulties for more of a challenge. Of course, the PvP version pushes everything to the max.
Even though you will invariably do a bit of soloing during your time in World of Warcraft, the game is a lot more fun when you have other people to interact with. Whether you’re starting out, or getting back into the swing of things, having helpful mentors feeding you suggestions (or gear, or gold) doesn’t hurt. The /r/wownoob subreddit is a good place to find friends to quest with, as is the main World of Warcraft subreddit (and don’t forget its helpful “Murloc Monday” thread for newbie Q&A).
A "stat squish" and "item squish" was implemented to lower the numbers used in the game, e.g. a legendary item previously with a level of 1000 reduced to 265. Unique class-specific buffs are added back, i.e. mages' Arcane Brilliance and priests' Mark of Fortitude. Titanforging—a random event that raises the initial item level of an item gained via drops or rewards—is still in the game; however, if the item is one of those affected by the Heart of Azeroth, then that item cannot be titanforged.
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.
Just like player item levels, the game's professions have become pretty bloated over time. Blizzard has made some updates to the leveling process before but Battle for Azeroth features a skill squish that should smooth things out considerably. Instead of having to level hundreds of points before you can start crafting Battle for Azeroth items, each expansion will have its own separate skill tier. In other words, you can start crafting Battle for Azeroth content right away, while the gear and items from other expansions will each feature their own separate counters. Your first Battle for Azeroth item will put you at 1/150 on the counter for the new expansion, no more grinding needed.
But compare that with BfA's lackluster prepatch. Last week, players' uber-powerful Artifact weapons, which they had spent the entire Legion expansion building up, abruptly burnt out with no in-game explanation, no storyline, no quest, no Dad jokes from Khadgar, just a line of text on the Artifact screen saying they had been sacrificed to help stop the destruction of the world--part of the ending quests for Legion.
We’re also big fans of Simple Armory, which gives you a similar look at your overall progression acrosss the entire game, as well as a gorgeous, detailed graphical listing of all the achievements you’ve unlocked and all the mounts, companions, pets, and toys you’ve collected. The site makes it incredibly easy to see what you still have to get, if you’re a completionist. If you’re looking to fill out your collection of mounts, the site will even create a handy, optimized guide of how you should spend your day (or week) adventuring to acquire them all.
Class abilities were reworked dramatically across the board, with some still needing a serious balance pass. That left many players not only feeling underpowered, but having to learn how to play their characters all over again. Bosses in large-group raid dungeons that were easily killed each week by experienced guilds of players suddenly became challenging again, or worse, completely unkillable, halting forward momentum on old content for a good chunk of the high-end player base.
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.