Use http://scholar.google.com to search your field. Usually papers with the highest number of citations appear first.
PS. 1. Your Question appears in the topic"Research papers", but your question relates to searching, and citations. 2. It would help us if you identified your field as pharmacuticals. 54 other people may have found your question too simple or too general to be answered. They may not agree with your proposition that number of citations = best.