Pick druid, paladin or monk. Get heirlooms if you can. Grind mobs until level 5 - 6. Start doing the easy "just kill these mobs outside town" quests until 15. Queue rdf as every role and do every dungeon you can (with heirlooms you could probably solo the dungeons) while you travel to higher lever zones. Keep doing the simplest quests while you're waiting for rdf (which shouldn't be long). I leveled a druid to 100 once doing this in maybe a day /played, but he was also rested for most of the process.

Make sure you have the two Flight-Path toys purchased, as this can save you time. It won't have EVERY flight path, but will have major locations for faster travel if you need to switch zones entirely(which you will). If you buy the toys on one faction, it unlocks for the other faction as well. So don't spend double the gold! These toys can be found at the heirloom vendor(Undercity for Horde, Ironforge for Alliance)
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
While this income stream dried up when Legion came out in 2016, Draenor garrisons multiplied the total amount of gold in circulation many times. During the Mists of Pandaria expansion, you were rich if you had a few hundred thousand gold but, after Warlords, rich meant being a multimillionaire. However, the impact of this extra gold was blunted for a while, because WoW Tokens were redeemable only for more game time until February 2017, limiting how much gold rich players were willing to sell.
As always, I will monitor and update this guide as new information comes in. If I've gotten anything glaringly and completely wrong, just tell me with as much data as possible. The idea is to find the fastest possible leveling method, NOT to argue about how good or bad individuals are. If you have hard data, post it! If you have an idea, let the thread know so we can test it!

First off, there is no reason to ignore the war Campaign, you need to do them to unlock world quest. Beside that, the way scaling works, war campaigns are easyer to do while leveling then at 120. Everything becomes alittel harder at 120, untill you get some gear, because of them scaling levels. Also, i did my leveling on a demon hunter in about 8-9 hours. I think i know what youtube video you talked about, and i strongly dissagree with him. One thing tho, total xp needed was buffed with 15% that in worse case would make me take 10hours, the way i did it.
Fourth: When the enemy faction controls the Warfront, you get access to 11 quests that allows you to trade materials for AP and honorbound rep. This is huge because you get 750 AP per turn in and 500 rep per turn in which gives you a total of 8250 AP and 5500 rep; that is a massive amount. All the material you got from scrapping all your leveling gear will come in handy here and I imagine that you will be able to trade other players for different turn ins. I highly recommend trying your best to do all 11. Also, this will be a great opportunity to make some serious cash for your profession because people will be buying the same items in bulk.
I just decided that I would simply finish leveling my hunter the hard way, but when i can get heirlooms ill make a warrior and follow your guide to 80. I definitly want an 80 warrior for cataclysm... seeing as worgen warrior is going to be a crazy tank! By the way, is using heirlooms with your guides ok? And what class should I be for a tank? I will obviously be switching to worgen anyway but i would prefer best tanking until then. Thanks Jame!

Si vous êtes du côté de la Horde, Nous vous recommandons fortement d'également prendre 40 Kit de planeur Goblin, pour vous faciliter la vie. Les consommables ne sont pas très cher en ce moment et il est donc conseillé de faire vos stocks. Vous pouvez également demander à un mage de vous fabriquer un peu de nourriture avant de débuter votre grinding, afin de ne pas gâcher votre propre nourriture.

Avatar – Avatar over Deadly Calm. Deadly Calm seems great until you use it and realize the spells you would like to spam either have cooldowns or don’t even cost rage. It mainly allows you to spam Slam for 6 seconds but I usually have enough rage to spam Slam anyways. On the other hand, Avatar’s 20% increased damage is always reliable and useful at anytime.

In the end leveling quickly in World of Warcraft is a lot about experience and leveling you 2nd or 3rd character will always be quicker than leveling your first one, no matter what leveling guide you use. We at FMG hope you find our World of Warcraft leveling guide useful and if you need help with anything or have comments about the guide, leave us a comment here and we’ll get back to you!
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As you're leveling, I generally recommend you just follow the natural flow of the progression through each zone. They're laid out fairly reasonably. But don't be afraid to hop around, especially if you're near a quest objective. You shouldn't actually need ALL of the zones listed to reach 62. Pick the ones you like best. It's much more important to simply be as focused as possible and flow from one quest to the next.