Objectively, I would say that Text Response would generally be the 'hardest', followed by Context, followed by ULP. As Polonius mentioned, Text Response is quite a lot of work, and in ULP, you just have to demonstrate skill, as opposed to demonstrating textual knowledge or ideas etc.
For me personally, Text Response and ULP were the most enjoyable/easiest by far. I wasn't as motivated for Context because of my choice to write short-stories - it was a gamble, because hitting the criteria becomes more subjective that for another style, and my teacher was quite cautious in the way he encouraged us - he always wanted me to do expository. I also didn't think that practice would help me very much for Context - every story is hugely different from the other, so I really just didn't see the point in practicing when I already knew I could generate a story well within the time limit (and well under it come exam day, O, the horror!) So Context was the biggest 'uh oh' for me walking into the exam, because I knew I could score well in the other two sections, and for Context I was just like "well, I hope you write something good when the time comes."
The subjective and abstract nature I think also makes it the hardest intellectually. Like, Text Response requires more effort imo, but I would much, much rather teach a class full of people Twelve Angry Men than Context.