For Horde, Vol’Dun the area captures the feel of Battle for Azeroth is both the hardest and continues than other areas in the game. It can get quite frustrating in this area so better to go it early when you can utilize your old gear. The next two areas are highly dependent on personal opinion – Nazmir is filled with mobs and Zaldalar is one of the easiest zones.
Unfortunately, the BfA prepatch took away more than usual--having an entire expansion balanced around an ultra-powerful weapon that abruptly turned into a dead stat stick made the end of Legion particularly depressing--and offered very little in return. The patch didn't flop because of bugs; it failed because it's boring, unrewarding, limited, unambitious, and actively feels like it's punishing players at a time they should be celebrating.
Was doing it at 52 on my lightforged paladin and it was easily the slowest zone out of the ones I'd done. Granted, I didn't have War Mode on so I didn't have any PVP talents, but it felt like about 40% of my time was spent waiting around for respawns or wandering around looking for mobs to kill to get a quest item that had like a 5% drop rate. Maybe I just got unlucky but I definitely won't be going to that zone again for future heritage armor runs.
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.
If you already have a high level character or a friend willing to give you gold then you can purchase Heirloom items. These items are bound to account and are accessible through the Heirlooms tab using Shift+P. The benefits of these Heirloom items is that they scale with your level keeping your gear up to date without you having to worry about upgrades, and they provide a significant XP bonus.
Tiragarde Sound and Stormsong Valley are both fairly similar zones, but Stormsong is a little longer, meaning that you might not even have to finish it to hit 120. Tiragarde also comes with a bit of a warning. There are plenty of side quests to do in the zone after you finish the story quests. Don’t do them. It’s not worth the time or the experience you’ll get.