30 Days of Dr Who - Day 16:
The conclusion of Journey's End, hands down. Donna nearly died already, more than once, just in those events, but she made it, she survived and thrilled and even actually did the most to save everyone and then...she has to forget it all. Not just the Doctor-Donna bits, not just that major event, but everything. Everything about the Doctor. She thought by becoming Doctor-Donna she'd be the most capable companion The Doctor ever had (as did I!), but instead she was a risk to herself because of all that knowledge.
It was so unfair and depressing and its a terrifying thing to think of: You gain all this wonderful knowledge, you've experienced all these different things that have changed you in wonderful and irrevocable ways, gave you a confidence in yourself you never before had, and suddenly it's all ripped away - and by the person you trusted the most, the best mate you thought you'd be with forever!
AND The Doctor - poor guy has to, essentially, forcibly erase one of his companions minds while she's crying and begging him not to...and will then have to avoid her from then on, taking care not to have her memories of him triggered lest it kill her.
...It's all just so...Tragic...
(Naturally the amazing acting by David Tennant and Catherine Tate sold the whole tragedy beautifully as well.)