As the crew of the USS Enterprise-D explored the galaxy on Star Trek: The Next Generation, they encountered some incredible characters who only appeared once. Led by Patrick Stewart’s Captain Jean-Luc Picard, the characters of TNG have become one of the most beloved science fiction casts of all time. As they went on new adventures each week, Captain Picard and his crew made even the weakest episodes of TNG worth watching. However, some of TNG’s best episodes became classics because of their guest stars.