THIS IS NOW NERFED SADLY. DUNGEON COMPLETION BONUS EXPERIENCE IS STILL THE SAME THOUGH
My leveling strategy since 7.3.5 has been using two accounts.
I bought a second legion account on the cheap from Blizzard for 25ish Euros, includes one month of game time and a lvl 100 boost.
My strategy has been questing to level 15. Then spamming Stockades to level 45. Then Zul'Farrak to level 58. Then Hellfire Ramparts to level 83 (gets slow after level 80 but I can deal with it).
I do Hellfire Ramps to 83 because that's the end at which doing Ramps is not worth it anymore, before hitting 80 you get about 40% of a full level from a full run. Between level 80 to 83 you get about 20%, when you hit 84 the mobs level turn green so a full run is about 10% at best. The bonus dungeon completion exp from Hellfire Ramps stagnates after level 78ish so it get's slower and slower after that. Same applies to Stockades, at about level 40 the dungeon completion bonus stagnates significally so you're better off hitting Zul'Farrak.
At level 83 I do the method zero* in Lost City of the Tol'vir because my level 100 boosted monk cant clear the instance fast enough to make it worthwhile, the mobs scale to level 90 and can be a hassle to solo on a level 100. Bosses can take up to 2-3 minutes to kill. By leaving my low-level character at the entrance and killing the last boss on a geared level 110 granting me the dungeon exp completion bonus is far quicker than clearing the whole thing on my 100 monk and following on my low-level character. The ultimate method would probably be if you had a second account with a decent level 110 that can plow through the whole dungeon while your low-level character is following. I don't have this option sadly.
I do this to level 92ish. Then I do questing, picking up treasures and bonus objectives in Draenor to level 100. Eight levels in Draenor is done in 4 hours.
Levels 100 to 110 is done by a mix of legion invasions, dungeon boosts from 101 twinks and bonus objectives. My 100 to 110 record is roughly 5 and a half hours.
I've leveled 1 to 110 in roughly 1day and 6 hours played (Priest), I was still trying to figure out and refining my method with this character so I wasted some time trying to switch between dungeons maximizing dungeon quest exp but I found out that it wasn't worth the travel time between dungeons and figuring out how to clear the dungeon in the fastest way possible.
I managed to level a Void Elf (hunter) from 20 to 110 in just under 24 hours played.
If you're thinking about getting all the heritage armors I'd suggest you try my method before it gets nerfed.
Estimated leveling times:
1-60 > 6 hours (takes longer IRL hours because you need to wait 1 hour after 10 dungeon clears in Stockades) Zul'Farrak can be done back-to-back because it takes about 7 minutes to clear so you won't hit the 10 dungeons an hour cap.
60-83 > 5 hours (takes longer IRL hours because you need to wait 1 hour after 10 dungeon clears in Hellfire Ramparts)
83-92 > 5 hours (takes longer IRL hours because you need to wait 1 hour after 10 dungeon clears in Tol'Vir)
92-100 > 4 hours
100-110 > 5.5 hours
* = Method Zero is when you use two WoW clients and you log into your account on a second WoW client and log into a high-level character while you leave your character that is getting boosted on a comfortable spot in a dungeon that will grant you exp for the kill. By doing this you can be "logged in" on two characters on the same account at the same time. Because by force quitting a WoW session on a character that inside a dungeon will take about 1-2 mintues before the game registers your character as offline.
Edit: Just to clarify, when I do the Tol'vir dungeon I clear all bosses and skipping trash on a level 110. When I reach the final boss I log my low-level character and park it inside the dungeon at a safe distance. Then I quickly log back on my 110, kill the last boss, get the bonus exp and then exit the instance. Rinse and repeat. The bonus exp is about 90k with looms.
Method Zero explained by Archvaldor: Wow Leveling Exploit Guide 7.3.5 1-110, 1-60, 60-80, 80-90, 90-100, 100-110, Fast Power Leveling - YouTube