3x04 Children of Earth: Day Four
An outstanding episode on every level: scary, exciting, surprising, funny and heartbreaking. Torchwood has really, finally come into its own with this miniseries (or maxi-episode, as I heard it described today, and it seems an even more fitting term). While it might be nice to see a return to 13 individual episodes next year, in many respects this format is even more exciting. Lord alone knows how they could top it though!
[Watch it (again) on iPlayer.]