Is it best to read an English book or watch a movie?

Why not both? Try each of them. If you don't learn anything on one thing as much as the other one, then it's not the best. Only your mind can know. Think.

It depends on one's hobbies.

It depends on which medium you are more comfortable with and what type of learning suits you best. Reading books need more focus and concentration since you have to rely on your own to understand the text, whereas when you watch a movie, the context or message can usually be relayed as the story progresses or how the actors portray the scenes. There is really no best way; it's just a matter of finding which one works best for you. Good luck and have fun! :)