After playing this game for so long, I've learned to take these kind of threads with a grain of salt.
When playing TBC, people swore that Vanilla was infinitely better.
When WotLK came out, people swore that TBC was better and the content was just an AoEGrindFest.
When Cataclysm came out, people swore than the content was too hard and said WotLK was better.
When MoP came out, people DESPISED the daily grind. They said Siege was horrible and lasted too long. They said it was the worst expansion to date.
Lastly when WoD came out, people said the leveling was too easy, the raid was a joke, etc. etc.
My point is just this: every expansion will hail older ones as the best, when at their present time they were being hailed as the worst. Five or six years from now, the same people will who are bashing Legion right now will be saying it was the best thing since sliced bread, all while bashing the current expansion.