I want to know whether is it ethical to express thanks and gratitude to a reviewer in the reply to review text. I feel we should express thanks to reviewers who spent valuable time to review our work.
It is not a question of ethics, it is a question about being courteous. Reviewers spend time to read and comment on your manuscript. Regardless of the type of comments you receive there will be significant voluntary work involved. It is only fair to show some gratitude. One way to think about this is that it is no different from any other type of professional correspondence.