I can’t pick just one.
Grizzly Hills for its absolutely beautiful soundtrack. It takes me back to the days of Wrath, doing Argent Tournament dailies. I miss those days.
Zangarmarsh, because it’s definitely the most alien place in Outland. The fungus aesthetic is perfect.
Dalaran. Especially when you return there during Legion. The city design and the music were always great, but the warmer and more relaxed tones in the Legion Dalaran has a “welcome home” vibe to it. Like the devs almost remembered when WoW was at its prime (Wrath) and wanted to welcome you back to those days.
Suramar, because if there was ever going to be a high fantasy Assassin’s Creed game, I’d want it to take place in Suramar.