Sometimes, i get confused when somebody ask me "How do you do"?
What do you think the best reply is.
I agree with mask.
The best reply to "how do you do" is simply "how do you do". It is a formula rather than a question that will accept different answers.
As far as I know, it is a formal greeting used when you first meet someone.

The best reply depends on how you are doing. If you're doing well, say "very well thank you". If you're not so good, say "not so good". It depends !!
Teachers: We supply a list of EFL job vacancies
Usually, people just answer "How do you do?" back. A bit strange actually.
 miriam's reply was promoted to an answer.
I disagree, you'd give an answer (usually fine, even if you're not) and then respond with "and you/rself?" or "how do you do?".
Students: We have free audio pronunciation exercises.