In writing a paragraph is it best to write in chronological order or in order of importance?

Yes! So long as you use some kind of order, your paragraph will be fine. Using chronological order is good if you are writing a paragraph to explain the steps for doing something, or to describe an event. Using order of importance would be good for writing a paragraph about different groups, or judging which thing is best. It is best to write a paragraph using some sort of order - it does not matter what sort for most paragraphs.