2005- The Christmas Invasion
I loved seeing Harriet Jones again, we will always know who she was. I didn't care for the direction her character went, but I still appreciated her, especially in her next appearance. My other favorite scene was The Doctor picking out his new clothes. This is always fun, and we saw some nods to old Doctors and companions hanging in the wardrobe. But the wardrobe room itself was worth it, the first room outside the console room to be seen, and only other until The Doctor's Wife. To be technical, we didn't see any new rooms until Journey To The Centre Of The TARDIS.
2006: Runaway Bride
Besides The End of Time, Runaway Bride is my favorite pre-Moffat era Christmas special. A lot of that could be my huge love of Catherine Tate. But aside from that it was fun. Seeing the viewpoint of a companion who found herself unwillingly in the TARDIS, as portrayed by Ms. Tate, was a riot. One of my favorite scenes is her first after finding herself inexplicably beamed into the TARDIS. Which is not even a proper word!
Donna struggles to get back to the church she had been getting married in. She's not impressed by The Doctor at all, something new, and she's also not carrying any money. (pockets) My favorite scene of the whole episode comes after she finds a cab driver willing to giver her a lift.
They really played around with the TARDIS in this one, and I don't just mean going back to the creation of the Earth. My other favorite scene is when The Doctor uses the same Huon particles in Donna that pulled her into the TARDIS to bring the TARDIS to them.