It's a loaded question really. There can only be one, as far as I'm concerned... the moment the TARDIS de-materialised around Sally Sparrow and Larry, leaving them surrounded by the Weeping Angels. I just about had heart failure the first time I saw that. One or two similar moments (Sally and Larry looking up to see the statue "coming towards them" after watching the DVD message in its entirety comes a close second. Feel free to add more.