That tis an easy one - Mount!
But you can only hav 3 on any char. So this guide is abit misleading to say the least.
Imagine grinding for heart of azeroth.
Thankfully, I care about neither :P Dont get me wrong, usually I crave some mount-hunting. But since the island expeditions are basically a roulette and since the whole artifact neck is a lot worse than our artifact weapons, I can safely save my ? coins and not give a fack. I sincerely hope blizz will finally step away from their new "philosophy" because this expansion had some of the worst implementations yet...
"Save them until Patch 8.3 launches to quickly reach Heart of Azeroth level 75 and unlock the third MInor Essence slot!" I'm sorry but I can't think of a more boring thing in this game. Grind your levels ASAP to get a new passive isn't exactly compelling, which designer looked at this and thought "Yeah, that's a suitable carrot."