2. Donna Noble, 10th Doctor's Companion
One of the only companions who never fell in love with The Doctor and was a perfect friend to him. She wanted to spend the rest of her life traveling with him and seeing all of time & space. Her outcome in the end is probably the most depressing and sad companion departure ever. After becoming Doctor Donna and become the most important woman in the universe, The Doctor had to erase her memory. It was tragic, sad, and horrible because you saw the anguish and sadness in his eyes, all while seeing how badly Donna wished it didn't end this way. Donna would be the friend who you could learn a few sassy things from. She'd be by your side forever.