Objectively, Legion is the best expansion to date and by a wide margin. RNG is too random which hurts raiding, Legiondaries are too imbalanced and WPvP is a complete joke. That is pretty much the only bad I can find with it. The theme didn't personally grip me as the Broken Isles retcon was a bad one. Gul'dan summoned it up from the ocean floor and it was supposed to be tiny.
Subjectively, Cataclysm was the best expansion in my eyes. If Dragon Soul wasn't possibly the worst raid tier of all time, people wouldn't remember it in such a poor light. The grind, the rewards and the difficulty was absolutely perfect. Did I mention it introduced transmog? Yeah...
The most overrated expansions was WotLK. Yep, I said it WotLK fanboys. The story basically wrote itself. It was the spiritual successor to WC3 which made it easy to make for lore. WotLK had AoE spam dungeons, is when LFG was introduced (forever destroying the community), when welfare epics became a thing and put a massive amount of resources into designing vehicle mechanics. It's when the game completely began to cater to casuals. Naxx 2.0 was a complete copy/paste from the original. For those that didnt do Naxx in vanilla, it was pry great, but for those of us who did it was lazy and stupidly easy in comparison. Ulduar was the greatest raid ever. Argent Tournament was a joke. ICC lasted too dang long. Oh, and DKs were broken OP the whole expansion. The reason people are nostalgic about it was the lore, simple as that.