Arms is the best leveling spec for Warrior in Battle For Azeroth. Fury is a great single target spec, but lacks the AoE needed to compete with Arms. You will be striving to fight 2-3 enemies at a time for optimal results and Arms has the perfect spells and talents for the job. My tests have shown the average amount of mobs you will fight at once is around 2.3, which makes Sweeping Strikes amazing for leveling. Perhaps my favorite combination in the game is Avatar + Ravager + Sweeping Strikes. It slices down large packs of mobs in seconds.
Edit 20 It’s been a week since this post and a lot of people are still reaching out to me with questions which is fine. I have had a few different people reach out to me to help make YouTube videos based off this guide and more info in BFA which I’m more than willing to do. Whoever gets the videos up first I will link them here for you guys to enjoy.