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.
Vol’Dun is the hardest and most tedious of the Horde Zones in Battle for Azeroth. It’s also probably the one that makes the most sense. Each sections of quests leads directly to the one you should do next. For all of these reasons it’s going to be best to get this one done early. Thanks to any legendary items you might have collected in Legion, you’ll be strongest in the leveling process before 115 and weaker after — because that’s when the legendary effects stop working.
You earn a large rested experience bonus when you log out in this rest areas. This gives you a massive 200-percent bonus to experience gain from most actions (though experience from quests isn’t included). The amount of rested experience you earn increases the longer you’re logged off, and your experience bar will be blue (instead of purple) while you have the bonus.
If you already have a high level character or a friend willing to give you gold then you can purchase Heirloom items. These items are bound to account and are accessible through the Heirlooms tab using Shift+P. The benefits of these Heirloom items is that they scale with your level keeping your gear up to date without you having to worry about upgrades, and they provide a significant XP bonus.
Incursions are a new world content activity that are unlocked when one character has unlocked World Quests. This feature is unlocked account-wide meaning that all characters on that account can access Incursions even without being Level 120. This provides for a lucrative experience acquisition method that is available in a similar pattern to what Legion Invasions offered with nearly an entire level earned by completing an Incursion series which involves four World Quests and one mini-scenario at the end. These also grant item level 370 weapons and Azerite armor, so they should be done any time they are up. Incursions follow a similar schedule to Invasions and are available for 20 hours once they are initiated.
Meanwhile, Tyrande Whisperwind and Malfurion Stormrage grow impatient regarding liberating Darkshore from the Horde's control, ultimately choosing to begin the offensive on their own, against Anduin's advice. In order to maximize their chances, Tyrande undergoes a ritual to turn herself into the Night Warrior, the avatar of Elune's most wrathful and warlike aspects. To counter this, Sylvanas orders several Night Elven heroes who fell in the Burning of Teldrassil to be raised as Dark Rangers to bolster her forces.
If you love running old content for transmog items, you'll love the new legacy loot system. Whenever a player enters a dungeon or raid where they are 10 or more levels above the content, the dungeon or raid bosses drop an amount of loot equivalent to what would have dropped for a full group when the content was current. That means players could earn loot for five people in dungeons and loot for 20 people after soloing a raid. The legacy loot system also drops other gear types, like cloth dropping for a plate wearer. It's now much easier to get the transmog gear you want, and you can also collect gear for your alts on the same run.
Tiragarde Sound and Stormsong Valley are both fairly similar zones, but Stormsong is a little longer, meaning that you might not even have to finish it to hit 120. Tiragarde also comes with a bit of a warning. There are plenty of side quests to do in the zone after you finish the story quests. Don’t do them. It’s not worth the time or the experience you’ll get.