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.
These quests for the amount of time to complete them are not worth a spot that a green quest and above takes up save for questing achievements - this can be finished up later at higher levels (such as for the Loremaster achievement) - or for later rep grinding. Do green quests as soon as possible to avoid the drop in experience when they turn gray.
If you level with friends Prot would probably be the way to go so you could pull and face tank the packs. I leveled 2 pallies both ret on beta and had 0 issues. Selfless healer and wake of ashes work absolute wonders. I leveled a bit as prot and just felt like I did 0 damage. I leveled solo so I don’t know about the xp fall off but I would assume it’s the same as legion.
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The biggest backdrop of Battle for Azeroth is the budding of a war between the Horde and the Alliance, two opposing factions that have more or less been on the same side of various struggles for a number of years. The separation of the two factions was refreshing and reminded me of leveling through vanilla WoW with unique zones given to each side (at least at the beginning).
Island Expeditions are a new idea that digs into the island vibe brought in by Battle for Azeroth and allows players to team up in groups of three to venture out onto various islands to gather all-important Azerite while fighting against enemies on the island and completing quests. The twist is that Island Expeditions are also a race against the opposite faction — whether it's three AI characters or three real players is up to you — but you may have to do a little player vs. player (or fake PvP) if you happen to run into them.
Again it breaks down to personal preference. I didn’t have an issue doing them at 120 on the toons I did them on. I felt like they took you out of the way when I just wanted to finish the zones. Too each his own. I liked it because I would finish the quest then be right there at the hubs for the quest on the new continent after I’ve finished all the quest on the first continent.
While Leveling your characters in BfA you'll also need to complete the new War Campaign quests in order to unlock world quests and the ability to venture to the other factions contintent. We cover all of this in our War Campaign guides and with out new Guide Tabs feature you can have both Leveling and War Campaign guides loaded at the same time to easily switch between them.
I defintely think at 118 you should go and do all four of these parts. These are required to unlock World Quests and probably take about 60-90 minutes to get through. If you do these whilst levelling (especially all in one go at 118) then when you hit max level you're ready to go on WQs. You can go finish the zones quests at your leisure over the next weeks/months or never even finish them if you don't care about questing.
Congratulations on reaching Level 120! There are a myriad of new things to do. The first thing you should do is head to your faction hub ship where you have been doing your mission table quests and footholds to pick up and complete the quests necessary to unlock World Quests. World Quests will provide you an opportunity to grind reputation levels, acquire gear, and gather Azerite. There are tons of new things to do that we will cover in our Level 120 Battle Guide found here: Level 120 Battle Guide