From the good Doctor Who? This is a tough choice. While Sarah Jane (Elizibeth Sladen) was a very good character she was like the BBC scream queen or some kind of damsel in distress more often than not, and was woefully scripted in one of most important episodes in Who history: Genesis of the Daleks. But she did have some great ideas and managed to get herself out of a lot of scrapes by herself. Plus she recieved many treats in her debut, as she learnt the Doctor's first false identity (John Smith) in the first episode featuring a Sontaran, 'The Time Warrior'.
What little I saw of Jo Grant was amazing, as she was very intellectual and managed to grasp what the Doctor did and how he went about it. Out of all the companions she's very mature and is quick to understand travel throughout time and space.
Harry was my favourite male companion because he had some great sarcastic jokes and managed to make him more useful than others throughout the entire series. Again, I didn't see much of him but he what little I did was entertaining.
But my all time favourite will be Romana II, who Tom Baker would marry not long after they were both done with Who. Because of the chemistry they had in the series and the reincarnated Romana was equal in intellect (as well as stubbornness - she didn't let a little thing like slavery by the Daleks worry her), I would say she was the most believable of them all. Plus she was saucier than an explosion in a Heinz factory.