You all know there are four main skills when we study a foreign language, which are listening, speaking, reading, and writing. So what's the most difficult one for you? I read a survey and it says, listening & reading are the steps for learners to 'collect' information, and speaking & writing are the steps to 'produce' the information that you've collected from listening & reading. Thus, speaking & writing are more difficult than listening & reading. And according to that survey, writing is the most difficult one among the four skills.
Certainly each learner will find out which skills they are good at or bad at because the ability of each learner in each skill is different. For example, some learners can listen very well while most others think that's the hardest skill.
And I think each language has its own most difficult skill too. For most Chinese learners, say, writing is always hard because its alphabet is pictographic. And for most English leaners, listening seems to be the hardest one.
As for me, reading is the most difficult one. It's so hard to understand everything written by a native writer.