Even though not everyone loved Catherine Tate's Donna Noble when she first appeared on Doctor Who in the Christmas special The Runaway Bride, the audience grew to like her after she joined the Tenth Doctor as his companion.
Donna was different from the previous companions in the renewed Doctor Who series. Not only was she older than Rose or Martha, but most importantly, she didn't fall in love with the Doctor. In fact, Donna made it obvious right from the start that the Doctor wasn't her type. As such, the two went on to become great friends, and the easy dynamic between them led to the creation of some amazing quotes said by Donna on the show!
5 You're Not Mating With Me, Sunshine!
When the Doctor and Donna reunite in the fourth season and avert a crisis together, the Doctor lets Donna know he wants a mate. Unfortunately for him, Donna thinks he wants to mate with her and takes outrage at the news. They manage to clear the misunderstanding. Donna still lets the Doctor know there won't be any mating with her because he's not her type; he's too skinny.
Despite the initial argument, Donna and the Doctor end up becoming the best of mates, and unlike with other companions, the show doesn't have to create romantic tension between them in order for the relationship to work. The only time they kiss is when Donna is trying to shock the Doctor into expelling poison from his body which might sound strange but is close to an everyday occurrence for the Doctor.
4 There Is An Outrageous Amount Of Running Involved
Donna isn't the most athletic person considering by the time she reunites with the Doctor, she has a sedentary job and doesn't seem to practice any sport. Despite that, she does well when it comes to running for her life... most likely because she's motivated by her desire to survive. Living as the Doctor's companion does include a lot of running, and even though Donna goes with it, she still complains about it at one point when she mentions that the Doctor runs a lot.
If Donna was designing the recruitment posters for the position of the Doctor's companions, she most likely would have included this detail. Running for his life is something the Doctor regularly does, and it's only one of the things that make the show and its titular hero so unique.
3 Oi!
Some of the best TV and movie quotes are remarkably short. Donna had her own catchphrase that she used on multiple occasions throughout her run in the series. The word oi became her weapon whenever she wasn't pleased with something and felt strongly enough about it to point it out.
The most notable exchange that includes this word happened between Donna and the Meta-Crisis Doctor - the human Doctor who grew out of the Doctor's severed hand after Donna had touched it. Since both the Doctor and Donna transferred part of their genes to the human Doctor, he inherited Donna's way of talking, among other things. So they kept saying oi to each other when they started arguing, and the human Doctor was disgusted by the realization that he only had one heart. This Donna's quote became so popular among her fans that there's even a compilation on YouTube detailing every occasion when Donna said oi.
2 Give Me Pockets!
The first time Donna and the Doctor meet in the Christmas special, neither of them is doing so well. Donna is angry because she thinks the Doctor kidnaped her before she could get married, and the Doctor is sad because he has just lost Rose to another universe. Despite that, they're eventually forced to work together and realize they function well as a team. So much so that in the episode's finale, the Doctor offers Donna to travel with him, but she refuses (for the time being).
The episode manages to mix dramatic and funny moments. One of the funniest conversations happens when the Doctor asks Donna if she has a mobile phone, and Donna is quick to point out that she's in her wedding dress, and it has no pockets. She then tops it by adding: "When I went to my fitting at Chez Alison, the one thing I forgot to say is Give me POCKETS!" Even though the pockets would have helped, the good news is that the Doctor and Donna solved the crisis without the need for a wedding dress with pockets in the end.
1 Save Someone
The main reason why the Doctor needs the companions is that they keep him human, let him remember the value of human lives, and that sometimes it's worth it to fight against the rules of time and space. In the episode The Fires of Pompeii, the Doctor wants to leave, but Donna tries to persuade him to save at least someone... and she succeeds in the end.
In a way, Donna functions as the Doctor's human side, and she later goes on to become the Doctor's human side for real when the human Doctor is born, thanks to her. Donna is the Doctor's voice of conscience, and this quote well demonstrates that there's much more to her era in Doctor Who than just laughing and joking with the Doctor.