Level 1-10
Finish all quests in your starter zone, there you should get level 10.
Level 10-20
When you get level 10 you can go to Westfall, Quests in Westfall are easy. So go there.
Level 20-30
On level 20. you can take mount so no more running :) When you get level 20, you can go to Duskwood, grab every quest you can, the quests here are very easy and provide lot of of xp, so you'll only be here for few hours then move on.
Level 30-35
When hit 30. lvl, lot of people like to go to Strangelthorn Vale, I would suggest you stay away from Stranglethorn becouse of the horde. But if you want some pvp action go ahead, just keep this window open because you'll be looking here for your other options. You can go to Arathi Highlands, the quests are simple and there are plenty of them, so an easy 5 levels.
Level 35-40
You can go to Dustwallow Marsh now, there are lot of quests here and usually no horde, so the questing will be nice and easy here.
Level 45-50
You have 3 options here.
Felwood: There are usually a few horde running around, and not many quests to do, but yWinterspring will be very close to you, you'll need to go there once you hit level 50.
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Azshara:I havent seen lot of people doing quests here, but there are nice quests and plenty of XP, You can go to Felwood for some more quests, Winterspring also.
Searing Gorge: Easy 45-50.
Level 50-58
Winterspring: Grab all the quests in Everlook. Quests are easy, and XP is good
Blasted Lands: Try to take 58. level before you come here. There are few quests, but XP is good.
Burning Steppes: Tons of quests, no horde.
Western and Eastern Plaguelands: Lot of quests, but you will meet horde here.
Un'Goro Crater: Nice place to level, you can meet some horde here too.
Level 58-65
Go to the Blasted Lands, from the flight master run to the dark portal. Take every quest in Hellfire, Grab every quest in Honor Hold and have fun.
Level 65-68
Go from Hellfire to Zangarmarsh.
Level 68-70
Go to the Nagrand.
Level 70-75
Time for Northrend. Do every single quest in Borean tundra. Take all quests in Valiance keep and do them all, also you can go to coldara and do quests there, you will get few quests for Nexus.
Level 75-80
Go to Sholazar basin, you can find a lot of quests in this place, If you don't hit 80 here, you can go to Storm Peaks and do final quests.
Starter zones:
Night Elves: Teldrassil
Dwarves and Gnomes: Dun Morogh
Draenei: Azuremyst Isle
Humans: Elwynn Forest
Thanks for reading.