What's the hardest math to learn?

I'm curious, when it comes to the vast realm of mathematics, which branch do you consider to be the most challenging to grasp? Is it the intricate web of number theory, with its prime numbers and factorization mysteries? Or perhaps it's the depths of calculus, where limits and derivatives dance in intricate patterns? Could it be the complex geometry that bends our perception of shape and space? Alternatively, is it the abstract algebra that deals with structures and patterns beyond our everyday experience? I'm genuinely interested in your perspective on what constitutes the toughest math to learn and why.