Also get the Heirloom Mount. Being able to have basic riding speed from the beginning will save you a lot of time if you're starting from level 1. Allied races start at 20, so this is a non-issue for them. It's important to note that Pandaren can not use this mount until they finish their starting zone and pick a faction. Being Horde or Alliance is a requirement of the mount.
In my opinion, Zygor makes the best WoW leveling guides. They have been around for more than 10 years, which shows their commitment to maintaining and updating their guides to provide you with an optimized leveling path for any class and race, whether it’s Horde or Alliance. Zygor is an addon that includes a waypoint arrow and a separate window with instructions that, taken together, show you step-by-step how to level your character to 120, FAST. You’ll know exactly what to do at all times. It’s also very easy to jump in at any level; you don’t have to start exactly at level 1 or 110 or anything. It’s a bit more expensive than the other options, but I’d say it’s definitely worth it.
Heirlooms. At the bare minimum you make sure all the armor pieces upgraded to 110, since this is the lion's share of your experience increase. Neck and Ring upgrades are secondary, but still important due to being able to enchant them. Trinkets can't be enchanted, but you'll want them at max as well, if only to save time by not having to worry about replacing them as you level.
These two expansions are virtually identical in terms of exp/h. I personally favor Mount Hyjal, and think it's very slightly faster. But if you know Jade Forest well you can just as easily keep up there. Ultimately you'll end up doing both anyway, so it's mostly academic. Every time I've tested Valley of Four Winds or Deepholme, or any other zone in MOP/CATA's level range, they were worse exp/h. You should not even need to complete both zones to ding 91, so it's more important to just pick one and go.
This whole affair has caused myriad arguments with the community, with some old-timers noting that the time-to-level increase is a joke compared to Vanilla, and newcomers sharing their point of view that World of Warcraft is slower than a lot of other games on the market. I can certainly understand both mentalities, but given that the leveling system is sort of a token process now and is already frustrating for a lot of players -- seeing it shaken up this much can't be good for the game.
Special note: At 110 it's almost certainly worthwhile to go and immediately get your Heart of Azeroth and first piece of Azerite gear from the BfA opening quest. It doesn't take long and provides pretty high power levels. I'd then return to Legion quests until 111 or even 112, since the power of these items will increase your kill speed significantly over the scaled enemies of BfA content.
Congratulations on reaching Level 120! There are a myriad of new things to do. The first thing you should do is head to your faction hub ship where you have been doing your mission table quests and footholds to pick up and complete the quests necessary to unlock World Quests. World Quests will provide you an opportunity to grind reputation levels, acquire gear, and gather Azerite. There are tons of new things to do that we will cover in our Level 120 Battle Guide found here: Level 120 Battle Guide