Iron Keep is. Shrine of Amana and Gutter and Gulch are, in this order.
But Iron Keep takes the crown. After playing recently through it again, all doubt vanished. It is the most bullshit area in the game. Placement of enemies, number of them, aggro range, number of invaders and phantoms, the route you have to take including the super narrow one which lets you be flanked by NPCs from the front and the back with one turtle literally blocking your way out of that narrow passage, all of these make it the shittiest area in the whole series.
And you know what? The 3 or 4 zones that I mentioned above are mandatory, you have to go through them in order to finish the story whereas Frigid Outskirts isn't.
On top of that, Frigid Outskirts is wide open allowing you to sprint and better maneuver yourself during combat and you have 3 summons you can rake with you.
Iron Keep on the other hand? It's closed, narrow, whatever open area you get to is extremely dangerous because there are Alone Knights literally everywhere you look shooting at you from a distance, their armor textures help them camouflage with their surroundings making them harder to see from a distance, hell sometimes even from up close. It barely matters if you go slow or fast. I learned Speedrun tactics to get through that shit but even then it's a coin toss.
It's one of the places on DkS2 where the devs had no idea what to do due to lack of time or resources or whatever.
DkS2 is my favorite but these 3 areas, particularly Iron Keep, can go to hell and stay there.