If you’re jumping back in for Battle for Azeroth, we recommend checking out the big “what to do first” guide from Wowhead. You’re also going to want to brush up on specific advice for your class—both Icy Veins and Wowhead have incredibly detailed guides for all of the game’s combinations of classes and specializations. Make sure you go in and figure out all the new (or changed) talents you’ll be using, and start to memorize a typical combat rotation so you don’t die nearly as much in game’s latest incarnation.
Isle of Quel'Danas: Special mention. At lvl 70 you can start doing the dailies here. Despite not being able to fly, the quest density is VERY high. I recommend hitting this every reset if you spend more than 24 hours REAL time(not /played)in the 70-80 level range. In fact, if you're doing a monk and only logging in once each day anyway, IQD WILL be your best exp/h for this level range.
Some people have made arguments that quest blues can sometimes be more powerful than heirlooms, especially with sockets. I'm not convinced. The simple fact that you never have to even look at quest rewards will save a non-insignificant amount of time over the course of leveling. Also of consideration is the value of enchanted gear never having to be re-enchanted.
The expansion brings a major change to the PvP ruleset on each realm. Every realm by default only allows players to attack NPCs in the open world; players who wish to engage in world PvP now have a setting called "War Mode" that can only be toggled on or off in their faction's capital city (i.e. Stormwind for the Alliance and Orgrimmar for the Horde). While in War Mode, players have access to new talents and abilities, as well as a slightly accelerated rate of XP gain. Characters with War Mode activated are only able to see other players with War Mode, unless in their own capital cities.
Prepare the Horde for war by recruiting the ancient empire of Zandalar. In this troll-dominated territory, ancient evil waits to be unleashed on the world as you battle crazed blood-troll worshippers, gargantuan dinosaurs, and titan constructs. Discover Zuldazar, the oldest city in Azeroth; unveil the bleak swamps of Nazmir; and traverse the deadly deserts of Vol'dun.
What’s worse, it’s rarely satisfying when you do make it to a new milestone in your grind to unlock the gear you need. Legion had legendary items that changed the way an ability functioned, which would also completely change the way classes played; the Azerite traits in the Battle for Azeroth are rarely as interesting. Instead, they are simply passive bonuses that, in most cases, help you do more damage without any extra effort on your part. While some classes are lucky enough to have traits that change their gameplay slightly, Blizzard has so far seemed to favor nerfing those traits out of viability for most specs.
Anyway both got to 110 at around 50h played. I messed up with my Pal by going to Legion once I hit 100 instead of finishing treasures and bonuses in WoD. All in all, if I picked the best zones and was only trying to level as fast as I could, I would probably could have done it in roughly 40h. Note that it would have a been A LOT faster with a Monk as both Ret and Rogues don't really have a decent AoE until 40+ (45 for Ret and 63 for Rogue I believe). The daily also helps a lot. I know that leveling my Monk before the pre-patch was insanely fast. Might be slower with the changes to FoF, however.
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.