So, series 4 is the only full series of Doctor Who I'd watched (before the Eleven era) so I don't have much precedent to say this but, Donna Noble is my favorite companion. She is brash and honest, intuitive and kind, and she's a fighter. Donna never believed she was special but she was actually the most important woman in the world, and this makes her storyline the most heartbreaking I believe. She finally learned how special she was but was forced to forget. The saddest thing ever for me :(
day 4; Donna Noble (Doctor Who)
"What and that mean's arresting ordinary factory workers? In the streets? In broad daylight? It's more like Guantanamo Bay out there. Donna, by the way. Donna Noble; since you didn't ask. I'll have a salute.