Check out all parts of the leveling guide:
Part 1) Level 1 - 58 / vanilla leveling guide.
Part 2) Level 58 - 70 / The Burning Crusade leveling guide.
Part 3) Level 70 - 80 / Wrath of the Lich King leveling guide.
Part 4) Level 80 - 85 / Cataclysm leveling guide.
Part 5) Level 85 - 90 / Mists of Pandaria leveling guide.
Part 6) Level 90 - 100 / Warlords of Draenor leveling guide.
Part 7) Level 100 - 100 / Legion leveling guide.
Level 100 - 100The fastest way to level up in Legion is to rush the quests needed for the main objectives while skipping the optional ones. A very useful add-on called Conslegion will help with this, and will always tell you what quests to do, and which to skip, and makes leveling up a lot faster!
When selecting zones to level up in, go for either Azsuna og Val'sharah first, because these are fastest in terms of XP/hour. After doing these two, go for Highmountain, and try to hit 110 because even going to Stormheim (unless it's your first character, because then you want to do all zones to get friendly with each faction).
When you finish a zone you get a dungeon quest, and make sure to queue for this dungeon as soon as you get the quest. The experience reward is pretty good, and the items from them makes leveling up later easier.
At level 101 you should return to your Class Hall to start doing missions. At pre-110 XP bonus missions will be offered, and these takes 2 or 4 hours, and gives you 5 % of your current level as the reward. If you are not going to play it in one sitting, these missions really add up over time!
The XP missions costs 100 Order Resources each, so you might need to do some bonus objectives, kill some rares or loot some treasure chests in order to have the resources needed to go on these missions. If you run low I suggest running trough a whole zone while hunting for rares. They usually drop 70 Order Resources each, and getting a few hundreds is pretty fast.
If you want to do random dungeons while leveling, expect around 25 % of your level progress from each random instance. A lot of the XP comes from completing the instance, so don't leave until finished. Doing specific instances will give less XP, but if you have two or three quests in a particular instance, it will pay off.