Comparing EN with the most recent stuff
WOD-Highmaul I would say was infinitely better than EN. This is probably the closest though.
MOP- Mogushan Vaults, Heart of Fear, Terrace of Endless Springs. Wins by sheer amount of content, and while there were some awful fights (Lei Shei, Amber Shaper) there were also a lot of great fights.
Cata- BWD, TOT4W, BOT. Again wins by the sheer amount of content, also probably the most underrated and best tier we have ever had.
WOTLK- Naxx, OS, EOE. A lot of people bitch about Naxx, but to 99% of the playerbase it was unseen content. OS and EOE were short but sweet fights with Sartharian 3d 10 man being incredibly challenging early on.
Emerald Nightmare and why its bad. The textures and scenery are bland quite frankly, Xavius looks like a fat demonic mario brother. With Nighthold being 2 months away its basically a tier of its own.