For me, the best teachers were always the hardest. My favorite teacher of all time was my English and literature teacher in college. She would go over your papers with a fine tooth comb and make so many comments, she was so hard to please. When I got my first A paper from her, it was one of the best days of my life.
Least favorite was the easy ones. I had a teacher whose last name had three A's in it, and there was a good reason for it. You showed up. You sat through his boring lectures. He didn't grade papers, he would check length and then give you your A, and that was it. I don't even remember what the class was about.