(^Continued)
As for declaring a favorite companion, I would definately have to go with Donna. Of all the actresses that played the companions, Catherine Tate is the only one with the versatility and screen presence to be able to keep up with, and occasionally outpace, David Tennant. I also liked that Donna was the Doctor's equal. Rose & Martha were so awestruck by the Doctor that he kind of took over their lives & their personalities. They were in love but it was a very worshipful love, since the Doctor was clearly on an intellectual plane far beyond them. But while Donna could never hope to match the Doctor's intellect, she did become his moral superior. She was the one with the guts to stand up to him and point out how cold & callous he could be sometimes, or self-righteous.
They kinda did that with Rose a few times early on, in "End of the World" ("You were too busy thinking up cheap jokes about the Deep South!") and "Dalek" ("He's not the one pointing a gun at me.") but it didn't really take. Maybe she was just too in love with him. Maybe he guilted her into never questioning him again after her screw ups in "Father's Day."