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!
If you bought Legion and have no desire to go through old content, good news! The new expansion comes with a free boost to level 100 for one character. You can buy these boosts from the in-game store whenever you want, but they normally cost $60, so savor this free one. Use it for a class that you are confident you will enjoy. As a fresh level 100 character, you will be able to immediately begin the quest that takes you the Broken Isles, the new region where Legion takes place.
The guide should advance to the point where you are in the zone's progression. There are a few steps that can-not autocomplete and they should (hopefully) have comments to the effect of -- you need to manually check this step off. -- For some reason you have to click these steps off 2-3 times to work. (the first time will move the tom-tom arrow, the 2nd time unchecks it and the 3rd click advances the guide).
First, make sure you’ve performed a recent scan to ensure you have the latest data. Then, open the Gold Guide Menu by clicking the quick access button (looks like a gold icon) at the top right and when the menu opens select the “Crafting” section. Select an item you’d like to craft and open its crafting guide by clicking the orange button at the left end of the row. Most crafting guides require you to gather a list of ingredients and give you the option of either farming the items or buying them from the Auction House. With the guide open to one of these steps visit the Auction House and select the Buy tab. All the items you’re told to buy should automatically appear in the Shopping List.
Be warned that players will occasionally list items in the Auction House that are sold by vendors. This typically applies to limited sale quantity items or items in remote, harder to reach locations. Items from Outland that advance certain skills (such as books for training Cooking or First Aid past a skill of 300) are typical. These items are notoriously listed for 2, 3, or even 10 times their vendor purchase-able price. Using an add-on such as Auctioneer Advanced can forewarn you of such a tactic. You should ONLY buy such items if your server is so full and busy that you simply are unable to acquire the item on your own. Otherwise, wait for the vendor to re-stock (usually takes about 20 minutes to 1 hour depending upon the vendor). You'll save yourself a considerable amount of money. See Buying Items from Vendors for Resale below for more on this.
Even if you don’t have flying in WoD be sure to get the WoD handy notes. If you have flying simply fly and pick up all the treasures that you can and do bonus objectives. Not every single piece of treasure will be worth it though as some are hidden in caves or are just hard to get. So if you find yourself spending more than a few minutes finding a treasure, just skip it and move on, there is plenty of treasure to go around.
Many players who already have higher level characters create alts that they level to a certain point and then stop. Often, these twinks are level 18-19, 28-29, 38-39, etc. for the purpose of going to battlegrounds at the top of their tiers and kicking butt. Since these twinked characters are owned by higher level players with lots of cash, they usually outfit them with the best gear available at their level. Thus, items that require level 17-19, level 27-29, or any other items around this level, with good stats or dps, often sell for much higher prices than they normally would. This is especially true on an older server, and also especially true of blue (rare) items. In general, "good stats" include Cloth "of the Eagle" (for mages, warlocks), Leather "of the Monkey" (for hunters and rogues), and Mail "of the Bear" (for warriors/paladins)as well as weapons with these suffixes that can be used by the right class.