Sometimes is not the type of engineering but the project you are involved in!
I work with structural engineers, mechanical engineers, water engineers and depending on the project and its features, the structural side of it might be challenging but the water part might be the most time consuming. It all depends on the project!
For me computer software engineering is the most difficult. I am a mechanical engineer and this is not difficult for me. If you were to ask a software engineer he may say mechanical engineering is most difficult. I think each person you ask would have a different answer. What is most important is to pick the engineering subject you are best at.