This post contains spoilers for Ardenweald and Revendreth,
I loved Ardenweald. I've always liked the fae in fiction, and I think Blizzard did a great job with their version. The NPCs are great and likeable. The Winter Queen is appropriately powerful and mysterious. The story itself is excellent, especially if you played through Legion.
The quests with Ysera were very well done. I especially liked that, when you replay her death, your character shows up to attack, labeled as 'Hero'. I made sure to breathe fire on my character.
I really enjoyed playing through Ardenweald. It was my favorite zone this expansion. I'm probably going to go Night Fae for my covenant.
Revendreth is Victorian Gothic, with an interesting spin on punishment and redemption. It had a lot of interesting concepts, especially the idea of "sinstones" acting like true names and giving power over a character. It was also interesting that all the Venthyr NPCs, even the friendly ones, were once terrible people. The main villain was also quite good.
I liked Revendreth a lot. I'd rank it the second-best zone. Whatever alt I play next will probably go with Revendreth as a covenant.