30 Days of Doctor Who: Question 2: Favourite Companion.
I could make a case for each of them, but in my heart of hearts, my favourite companion is Captain Jack Harkness.
Because he made the Doctor laugh.
Because he was a con man who became honest. He was an action hero with a good mind.
Because he would, and could, and did give his live for the Doctor. And then lived on, no less loyal.
Because he had a vortex manipulator.
Because he was flirty, and smart, and resourceful.
Because he had a great coat.
...and he made me laugh.
"The extrapolator's working. We've got a fully functional force field. Try saying that when you're drunk."
This being said, on the subject of companions, I loved all the companions of Nine and Ten in various ways, and for different reasons:
Rose - Because I loved the way the character developed when she was with Nine: from shop girl to master of the vortex.
Because I love the way she fell in love with the Doctor, and got her priorities straight.
Because she was smart and unpretentious.
Because she accepted the Doctor as he was, even when she didn't undertstand him.
Sarah Jane Smith - Because she
did understand him. And because she was an action hero female in her fifties.
Martha Jones - Because she was clever, and beautiful, and brave, and kept surprising me.
Donna Noble - Because she became so much more than she started as - and I don't mean being a Time Lord, I mean being a companion who was equal to the Doctor.