7. Donna Noble (Journey's End)
Stepping aside the shocking nature of Donna Noble's first appearance in Doctor Who, her exit was particularly surprising not least because she got to do a pitch-perfect impression of Ten before she left. During the course of Journey's End, the Doctor's regeneration had partially invigorated her with Gallifreyan DNA, giving her heightened intelligence. This change had been foretold in the prior exit, but until it actually happened, we didn't know exactly what was going to happen. Her exit was a take on the earlier exits of Jamie McCrimmon and Zoe Heriot back in the second Doctor era. She basically had the adventures that she had with the Doctor wiped from her memory, lest they bring forth the Gallifreyan side of her and kill her. Ok, so McCrimmon and Heriot didn't have that bit in their exits, but they also couldn't remember their adventures. This was quite heartbreaking, as we'd seen Donna change during the course of the season and grow up. After the love story of Rose and the unrequited love of Martha, it was good to just step back from all the angst and give the Doctor someone with whom he could joke around. Finally in Journey's End, with her becoming the Doctor-Donna, she was simply amazing. So amazing that she was probably just too powerful for the series, and had to be cut down accordingly. She later reappeared in The End of Time which sees her get her happy ending with a wedding and a Doctor-influenced lottery win. It sort of wrapped it all up, but the original exit was still traumatising because it felt like all that good work was completely undone.