"The Empty Child/The Doctor Dances". It's a creepy story involving a creepy child, so that's awesome, and despite the darer tones of the story, it ends on a rather heartwarming matter. I love things like that.
"Bad Wolf/The Parting of Ways". For me, the first part of the story is just perfect, and while part 2 is a letdown in some regards, the Dalek's invasion of the space station is one of the best uses of the Daleks ever.