"What course did you like the most?"
"What was your favorite subject?"
When you answer this question, use a subject that helped you become a better person or a better worker. Although this question is asking for you opinion and there is no wrong answer, you should take advantage of this question by showing your strengths.
Short Answers
"I really enjoyed an English writing class I took. This class taught me to write more clearly and concisely."
"My favorite subject was Physics. It really helped my logic abilities and I use this knowledge to solve problems in a variety of ways."
"There was a speech class I took that I really enjoyed. It helped me to speak in groups better and I learned to speak professionally in front of an audience."
"My favorite subject was applied science. I really enjoyed learning different ways of applying science into ordinary things."
Long Answer
"I really enjoyed majoring in electrical engineering and it really helped me to prepare for my career, but the class I enjoyed the most is probably a couple of psychology classes I took. Although it didn't help me in any technical way, I learned a little bit about human behavior and learned how the brain works. I started to understand the reasons for my strengths and weakness instead of just knowing them. Also, it helped me to understand people who are different than me."
This is a simple answer but a very good one. An engineer is mostly technical and they might lack skills in human interaction. So an engineer saying their favorite course was psychology demonstrates that this person has both technical skills and the ability to work with others better. It lists several good examples such as understanding both strengths and weakness along with understanding different people.