New Power-Levelling/Gold-Making Method, Very Advanced, Very Powerful, Part 1 (Patreon)

Since the changes to dungeon boosting in 7.3.5. it is no longer possible to run a high-level player through a dungeon  and gift a low-level player xp from mob kills. You still get dungeon completion xp and because of that this is still the fastest default of levelling. 

But what if you could effectively solo dungeons efficiently at the right level with a twinked out character, leaving your levelling character at the entrance gaining full xp? Then you'd get the same benefit as you did before the changes and could steamroller through dungeons. Also you can make an absolute ton of gold as you could before the changes boosting other people-and unlike pre-nerf dungeon boosting you'd have no competition. 

To lock experience visit either http://www.wowhead.com/npc=35364/slahtz (horde) or http://www.wowhead.com/npc=35365/behsten (alliance).

Warning: This is not a trivial thing to accomplish. While testing these ideas and dying repeatedly I wanted to headbutt my screen a lot. You need best-in-slot gear, full enchants and crucially, a good understanding of boss mechanics. Post 7.3.5 soloing dungeons at all is difficult: doing it efficiently requires high-level optimization. However, that difficulty means that if you take the time you have a rare and valuable skill you can charge a premium for, not to mention being able to level your own characters at light-speed.

First, it is important to appreciate the scope of what we are trying to do here: at each bracket, 15-60 60-70 80-90, 90-100 and 100-110 we have to locate the best possible combination of class, gear and level at every individual level. This is a huge, probably impossible task, however we have some clues from the twinking community posts over the years. I won't burden this post with all the sources I used to compile it, but if you want to try theorycrafting this yourself bear in mind that you are mainly looking at pre-legion twinking guides when there was much more crossover between pve and pvp twinking.

Because of the complexity involved all of the below information is preliminary and subject to change:

15-60 I have had the best results at this level using a survival hunter, particulary their completely broken ability "Flanking Strike" which one-shots all but the strongest mobs. You will need decent gear in each slot and full enchants. I used elemental force for my weapon. Unlike the other brackets most of the power from this approach comes from simply being a survival hunter.

For Alliance you can either level 15-20 in Deadmines or just mob grind murlocs at crystal lake in Elwynn. From 20-60 do Stockades in Stormwind, the most efficient dungeon in the game.

For Horde just do Ragefire Chasm in Orgrimmar.

An alternative, much more  exploitative approach for intellect users is possible for Horde players. 

There has long been a known twinking exploit available at all levels which has never been fixed:

https://www.ownedcore.com/forums/world-of-warcraft/world-of-warcraft-exploits/323160-h-free-extra-60-intellect.html

How to use this: Take a high-level mage, or class trial mage if you prefer, and park them in Warsong Hold in Borean Tundra. Take your twink character to the same place. Clear out any mobs around the webbed characters with the high-level character, then log-in on your low level and free the webbed characters till you get the "Warsong Mage" NPC who buffs you with a 60 intellect Arcane Intellect buff which lasts half an hour. You can then set your twink's hearthstone to Warsong Hold and group with the mage using customg group finder to port back to Orgrimmar.  

Using just heirlooms you should now be able to solo Ragefire chasm on behalf of any of your characters or any one who wishes to pay gold for the service. You can repeat this procedure boosting a levelling character up till level 60.