As a Spanish speaker who mastered English over the years. I think that verb inflections, cases (nominativ, dativ, akkusativ, genitiv) and 3 genders for nouns are the hardest. Having to memorize 3 genders, write different articles for each (der,die,das,den,dem) depending on the verbs and other elements in the sentence could be such a headache...

This is just my opinion, of course. :)

It depends on the native language what a person finds the most difficult in the other language.

For me the word order in the sentence is quite confusing sometimes, verb forms and noun cases in the beginning combined with arlicles can occur to be a problem.

But nothing that you cannot overcome with practice and desire to learn.

Good luck!