I'm generally really reserved on saying things like this, but I've been thoroughly impressed so far by Legion's environments. Both the dungeons and zones of this expansion are, in my very subjective opinion, better than BC's, better than Wrath's, better than Cata's, and way better than MoP's or WoD's. We still haven't had a chance to look through Suramar on the alpha yet, which actually appears like it might be the most epic of ALL of them... and I would still rank this expansions zones and dungeons above all others.
What I think is really interesting about this expansion is they didn't seem to just try to come up with a fresh take on a zone or dungeon, what they did was sort of look back on all of the best parts of dungeons and zones in the history of WoW and combine them in interesting ways.
There's a taste of Karazhan in Black Rook Hold and The Arcway. There are hints of Azshara (the zone) in Azsuna and the Eye of Azshara. There are hints of the Howling Fjord and Icecrown in Stormheim. They've taken that really great feeling of an ancient and mysterious forest from Feralas and Ashenvale and recaptured it in Val'Sharah.
In all of the environments for the class quests you go back to these old places that just hit you with insane nostalgia. They've been forgotten, but all they really needed was just a little extra places added (Karazhan Catacombs) or just an extra little lore (ICC, Duskwood).
Legion is a love letter to the old Warcraft. They tried to make WoD that by actually going back in time to do it and it failed. But Legion is the old Warcraft's spiritual successor.
All that is left for me to see to say this is the best expansion in the history of WoW is to see how they handle the end game. To me that is all that is left to be proven. Will the outdoor adventures work? Will there be interesting daily/endgame questing? How will the dungeon reward structure hold up a month or 2 months in?
If they nail that - this will pass Wrath for me.