The 30 Days of Torchwood meme:
Day 9: A Scene that Made You Cry
The death of Toshiko in "End of Days".

Why? Well, first of all, because I loved Tosh; and what really got to me is that she didn't tell Owen she was dying. The capper was how Jack held her lovingly while she died. And Gwen, too.
Chokes me up just to remember.