My most favorite is creating characters and plot! For me, I try to think of my characters as real people, and so I bring them to life in my mind. And then when I'm writing the overall outline and plot details, I think about what they would do in the situation.
For me, my least favorite aspect of writing is when I'm writing a story and I realize that the plot is going way to fast, so I have to put in more dialogue, background information, character thoughts, etc. So that it goes at a slower pace, while again, making sure that it's not slow paced either I also hate describing the background.