For me, vocabulary and writing are definitely the easiest. I pick up on kanji like nothing. From context and from my knowledge of some kanji and vocabulary, I am often able to guess the prounciation of some of the more common kanji words.

Definitely for me, particles, verb forms, and grammar structure is the hardest for me. I can't construct sentences for beans despite how much instruction I've had. It's really an embarrassing point for me.