I actually had this assignment in an education class. At least in an ed class, they can presume that you will adopt the qualities of your favorite teacher, and see what type of teacher you might be. It bites you in the ass either way.
I didn't have a favorite teacher either. I did have one teacher who, because of the situation, provided me with a lot of material (we had a Taiwanese student who spoke no English and no one spoke Taiwanese). I embellished A LOT.
The papers were returned, I was asked to read my paper aloud, and because this teacher was SO exceptional, discuss her teaching methods. It was a nightmare.
My son did have a least favorite teacher, but he wasn't smart enough to figure to say that he didn't have a favorite teacheer he had a least favorite and write about her. It would have been a very colorful descriptive paragraph if he wrote about his least favorite.