At 98 you have the option to drop what you're doing and return to your capitol city to start the quest for your Legion Artifact weapon. However, I do not believe this is necessary to do right away unless you're struggling with kill speed in your current gear. I would recommend waiting for a natural pause, such as hearthing to repair and clear your bags. From 100 to 107 you won't even really fight mobs that often; only just enough to clear the way to any treasure you need to pick up.
Download the game right now, create an account (a new one; not your old one, if you were a previous player), and you’ll be able to level your character all the way to 20 for free. There are other annoying limitations placed on this free experience, but it’s still a great way to familiarize yourself with the world, see if you enjoy the experience as much as you were hoping, and get a feel for the different classes before you settle on a main one to play as for the next 100 levels’ worth of gaming.
There are other in-game guides out there such as Xcelerated, but it doesn’t even come close to what Dugi and Zygor offer. Although I’m obviously a fan of Dugi Guides, I have given a fair side by side comparison of each guide, based on facts and what is offered by each. I’ve included many screenshots to give you an idea of how each guide looks and works within the game. I will do my best to keep this as up to date as possible, with price changes, guide updates etc as I know how annoying it is reading outdated reviews.
Stormheim is the zone you want to start it. For some reason Legion treasures are not worth any experience like they are in WoD...except in Stormheim. I haven't been able to find out any reason for this, and it's possible it may get fixed/nerfed at a later date. But for the time being you REALLY want to jump from treasure to treasure to augment your quest experience.
Dugi and Zygor both had their guides updated before Mist of Pandaria was released and both guides are kept updated with all the patches and changes to the game. Dugi is way faster with updates and has a lot more. Zygor seems to hold out and then do a big update. With new content such as guides etc, Dugi is definitely leading the way with Zygor usually many months behind. Zygor now offers the same number of guides as Dugi but it took them a long time to catch up.
Meanwhile, in Zandalar, the Horde seeks to earn the trust of King Rastakhan so they can use his legendary Golden Fleet against the Alliance. To this end, they assist Rastakhan and his court in dealing with local threats in Zuldazar, fight maniacal Blood Trolls in Nazmir, and face off with serpentine Old God cultists in Vol'dun. Throughout their journey, the Horde gradually learns about an eldricth being known as G'huun, an artificial Old God accidentally created by the Titans and the patron deity of the Blood Trolls. The Blood Trolls and the Faithless Sethekk seek to free G'huun from his prison of Uldir so they can use him to rule Azeroth, and to this end they resurrect G'huun's champion, the C'thrax Mythrax, to destroy Uldir's seal. Rastakhan's own chief adviser, the prophet Zul, is revealed to be the Blood Trolls' secret leader and launches an armed revolt against Rastakhan. With the Horde champions' help, Rastakhan is able to defeat Zul and his forces, but not before Mythrax destroys Uldir's seal.
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.