Once again, as soon as you hit 70 - I recommend to go and learn faster flying. This speeds up your questing A LOT, and is worth the time and gold. It will cost 4500 if you go to the capitol, or 5000 if you prefer Shattrath/Dalaran, because you will not have reputation with them at that point. This price will sadly be above what a totally new character can afford, if you don't have any Main. But you should go and buy flying as soon as you have enough money.
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.
Blizzard changed the leveling experience at the same time those new characters became available. It used the scaling tech available in its later zones to make broad swaths of older areas available to a much wider level range, so that characters could choose what quests or zones they wanted to do and stay there, rather than out-leveling them too quickly.
Anyway, however you choose to get to the addon's menu, choose guide list and then pick the zone you want to quest in. There are some zones that will autoselect based on quests in your log, but not all guides or all quests are set up that way. (If you don't want to switch to a guide that auto-pops up choose "not now" or "never") As you can see in the screenshot, there are various categories of guides. Most max level content (such as Argus) is found in Achievements rather than leveling.
In Legion, some of the dungeons include the Black Rook Hold, a max-level dungeon in Val’Sharah that once served as an elven fortress but has been corrupted by a malevolent force. If you decide to quest in Stormheim, you can also try to clear the Halls of Valor, which lets you take on the Vrykul God-King and vanquish the pure Val’kyr that are superior to those found in Northrend.
The zone you start in isn't under your control, in as much as it's pre-selected for you based on your race. But for leveling purposes we want to make scaling work for us as much as possible. That means sticking to the lower level zones. Even though scaling will increase the level of the quests and mobs in those zones, the layout and design of them is built for a player with no mount. So you'll blaze through them and gain more exp/hour than higher level zones which assume you DO have a mount.
Dungeons still give too much experience. Yes, I'll get some hate mail for this. But due to the way dungeons -- especially at some levels -- scale, along with experience buffs, the fastest way from 1-110 now involves grinding one dungeon over and over and over again. It's the equivalent of the South Park "Make Love, Not Warcraft" episode, only instead of killing boars, you're killing endless worms in Ragefire Chasm or gnolls in The Stockades.
If you queue as a healer - take care to always have a stack of water of appropriate level. You can buy it at an innkeeper for next to nothing, but remember that it will become too low-lvl for you very fast, so you need to buy new water every 10-20 levels. Alternatively, you can buy a stack of every tier of water in Stormwind, and just always carry it with you. Another alternative, if you see a mage - always ask him (politely) to create a table for you. Grab 2-3 stacks so you have extra for the next runs.
":" - Any time a step ends in a ":" instead of a "." means that the next step is part of the current step you are on. This generally means the next step should be done while working on the current step. This means every "start working on" or "continue working on" will end in a ":". But this is used with other occasions as well, so keep this in mind.
Heirlooms. At the bare minimum you make sure all the armor pieces upgraded to 110, since this is the lion's share of your experience increase. Neck and Ring upgrades are secondary, but still important due to being able to enchant them. Trinkets can't be enchanted, but you'll want them at max as well, if only to save time by not having to worry about replacing them as you level.
Leveling in WoW is pretty linear and easy compared to most other Mmorpgs, and the in game quest mechanics makes the leveling process pretty fast. Our WoW Leveling Guide is the best and quickest free leveling guide for WoW. Just start in your race starting zone and make your way to new level appropriate zones as you level up. This guide won’t go in depth on exactly what quest to do when, but you really don’t need anything like that in WoW anymore. However it will take you through leveling all the way from level 1 to level 90. From Classic WoW, Through the Burning Crusade, Wrath of the Lich King, Cataclysm and Mists of Pandaria.
I used fully upgraded and enchanted heirlooms; missing only the fishing ring. Leveling from 1-20 took ~3 hours, at which point I immediately turned on War Mode for the duration. I rarely encountered enemy players, and when I did they almost always left me alone. But the few times I did get ganked, the total cost in time was insignificant. Maybe a total of 5 deaths from ganks in the entire 40 hours.