It's not a stupid question, it a big one. Doctor Who has been around for almost fifty years and has had lots and lots of "supporting characters." Are you asking about companions? Villains? Extras? All of the above?
Me, I think one of my favorite supporting characters was Ace. She was tough enough to not only use explosives, but she also made them. Ace could also be vulnerable, too. Especially when it comes to her past.