Vanilla: nothing was really hard, some were a bit annoying with the complex maps.
BC: Shattered Halls was not really bad, it's just most tanks sucked at multitanking.
However CoT1 was hell. Mobs CCing the tank, while being cc immune if I remember correctly. I've been there like 2 times, tops. It was just atrociously annoying. And oh boy, Magister's terrace. Big caster pulls, tricky bosses.
LK, Cata, MoP: really nothing. Cata was a bit hard at the start but that passed quickly. Maybe ZA/ZG were a bit harder than the rest. ohh and Deadmines2 with the arcade part that broke most pugs DD
WoD: Grimrail indeed. Some mobs were brutal to the tank and the narrow spaces were sooo annoying.
Legion: Vault of Wardens, bosses are considerably harder/more annoying than average. Tormentorum is one of the least enjoyable 5man boss ever. Kara was definitely the hardest before 7.2 (I haven't played this patch yet)