The Darkest Doctor Who Story - Torchwood: Children of Earth

With the 3rd season of Torchwood, a new approach was attempted and has become one of the most praised series in the Whoniverse due to that. Russell T Davies took on a bigger role than he had in the previous 2 seasons, as well as bringing in Euros Lyn, who directed episodes of Doctor Who such as "The End of Time", amongst other episodes, to direct each episode. The story follows Torchwood during a global extra terrestrial crisis over the course of 5 days. Becoming a mini series for this particular story.
Torchwood is often referred to as a mature Doctor Who, and whilst the first 2 seasons definitely attempt that (to varying degrees of success) this season specifically is often referred to as one of the darkest stories within the Doctor Who cannon.
A great cast starring John Barrowman, Eve Myles, Gareth David Lloyd and Peter Capaldi, with excellent writing and direction, creates a 5 episode event that is utterly unforgettable.