Problem with that though is EN stuff doesn't hit hard enough to measure a tankiness of a tank in dangerous situation and relevant gear. TOV mythic bosses hit hard enough it can be some measurement for the future, Nighthold and on. Atm EN is deep into farm territory for most mythic guilds (close to 5 thousand guilds killed mythic Xavius while only close to 300 killed M Helya and less than 2 thousand killed mythic Guarm) and those for whom it's still progress 1 week before NH opens, they aren't the guilds that finish content anyway and they usually have problems with executing tactics and not because "they play underpowered specs".
So let's say we talk about guilds for whom playing a good spec matters to some extent because executing tactics and playing your rotation isn't the biggest foe. Those have EN on farm, tanks go for dps or just slack, some raids are done just so joe schmoe can rank for funsies, content isn't challenging and doesn't force respect, I've seen at least couple of top guilds on my server (not super world top but those who have stuff on farm for long long time) wipe on mythic Xavius just because people don't give the boss any respect and try to rank so desperately they miss the designated dream pool (funny how 1 guild was streaming that Xavius shit fest).
You can't really judge if x tank class is OP or garbo from EN mythic because any tank can do it fairly effortlessly in the current average gear levels. Worth of a tank is only revealed on content where your survivability is challenged. Otherwise you might as well start the dps contest and we have tanks running with angerboda trinkets and dps-maximizing talents rather than caring about minimizing damage taken.
If we go back to when EN was challenging (some claim it never was), then it's pre nerf-buff reshuffle so we had first warriors then druids reigning supreme.
After the patch if we want to compare how tanks fare in the current state of the specs, TOV mythic is the only semi-relevant place to the discussion. That's why I assumed we'd talk TOV logs not EN. Since Odyn nerf that fight is a joke though, the only challenging part is for dps and healers to run left and right and stack / spread correctly. The 25% nerf to spear and other nerfs makes tanking Odyn doable and fine as anything probably.
Thing is it's indeed hard to find proper evidence which tank is gonna be awesome and which is gonna be bottom tier until NH mythic opens because atm we lack proper encounters to throw the 7.1.5. edition tanks against. We'll also get set bonuses in play in NH and so on. Then there's also a question of utility, hard to gauge the impact of stampeding roar vs mass grip vs spellwarding vs monk statue and so on, that depends on fights design and tactics developed. Utility that ends never used because of lack of fights that call for it is good only on paper.
Survivability wise, druids have a very versatile kit and they would have to be completely butchered to actually stop being good. As long as they have spammable mitigation against physical damage, spammable mitigation against magical damage, self heal, decent rage generation and extremely powerful artifact ability they will never be "dead". I think the least versatile kit was designed for monks and death knights so depending on tuning they will either be weak or extremely good (if buffed out of proportions).