I think Exile’s Reach has to be the top one because it actually tutorializes a lot of stuff in WoW. If the starting zone is supposed to be for people who are starting out in WoW, that’s a great one because it teaches stuff like basic combat rotations, questing, vendors, dungeons, and innkeepers.
Beyond Exile’s Reach, I really enjoyed the Demon Hunter starting zone as a thematic introduction to who Demon Hunters are and their conflict with the Legion.
I think it’s a steeper drop-off after those two, but I did also enjoy the story in the Death Knight starting zone even if it’s very dated in terms of pacing and quest design (can’t forget the quest where you auto attack Scarlet NPCs for 10 minutes). And the Dracthyr starting zone is also fairly well done, with lots of good set pieces and an enjoyable introduction to the Dracthyr’s flying racial, Soar (which you will never use again once you enter the Dragon Isles).