The fav series poll had me thinking about what I liked in each one. At the risk of being obvious, I always found episodes with the Q entertaining and philosophically interesting. I also loved the Romulan "Romans in space" idea. The El Aurians were also interesting, though only vaguely explored in the series. I always enjoyed it when ST nudged up against demigodlike, or at least supernatural characters like Guinan, Q, The Caretaker, and various other non-corporeal beings.