7. K-9, "School Reunion"
A huge fan favourite and the only companion from the old series to appear on this list, the robotic K-9 has been a huge hit amongst fans new and old for the simple and undeniable fact that a robotic, talking dog is simply the coolest of cool. In his introduction episode alongside fellow favourite Sarah-Jane Smith, K-9 Mark III become a vital part of the finale, sacrificing himself to kill the evil aliens masquerading as school staff and teachers, eliminating a global threat at the cost of his own life. It's a heroic touch, one made sweeter when K-9 is rebuilt and goes on to aid in further adventures.