3. Velma Dinkley (Scooby-Doo)
Velma is the genius of the Scooby Gang. She is the one who tends to solve the majority of the spooky mysteries that the group so often encounter. For this reason, she would make a fantastic companion for the Doctor. Unlike Shaggy and Scooby, Velma is also quite fearless. She isn't at all phased by the regular bad guys that the group has to face - which are often ghosts and monsters (or at least seem to be at face value) - so the Doctor's foes wouldn't bother her at all. She's also quite capable of handling herself in the physical sense. In one episode, she was actually able to simultaneously lift a pair of muscular female wrestlers and throw them in to the air, above her head level, using one hand for each of them. She said that the move was called the "Flying Dinkley". If any of the Doctor's adventures required a mystery to be solved, she would add an extra pair of eyes and a useful extra brain that could be extremely helpful to him.