The one who wasn't: Rose Tyler
Controversial statement - Rose Tyler was way better with Christopher Eccleston than she was with David Tennant.
I know, I know. Think about it, though. In Doctor Who's first year back, we got what was a very new kind of Doctor/Companion relationship. This was the first time the companion really took center stage and the possibility at some romantic feelings was very different. And it all worked really well! The Doctor needed Rose to remind him who he was and what being the Doctor meant, and Rose needed the Doctor to remind her that life is more than just working in a shop and eating chips while your boyfriend watches the match. They grew together in such an interesting, but believable, way.
And then David Tennant rolls in and the whole thing falls apart. Rose and the Doctor suddenly become two schoolchildren taking bets to see if they can get the Queen to say her ridiculous catch phrase. What was subtle with Nine and Rose is writ so large once Ten comes along that you feel as though you're being beaten about the face with the will-they-or-won't-they of it all. They become so cavalier, in fact, that the all-but-entirely lose sight of the fact that they are putting people in danger while they're busy making googly eyes at each other.