My favourite moment of Utopia wasn't Jack returning, but
Spoiler:
the whole last ten minutes with the return of the Master.
Wonderfully done, and one of my all-time favourite cliffhangers. Truthfully, the only one to have matched it so far for sheer awesomeness just might be the cliffhanger to the Almost People. That was a classic moment of "HOLY SHIT!!!!"ness.

Series 3 of 8 and Lucie on sale today. Notable stories include:

Orbis (series opener)
The Eight Truths (part 1 of the finale)
Worldwide Web (part 2 of the finale)

Not so many arc heavy eps this series, although there is a stronger theme of development between Eight and Lucie's relationship due to the events of the series 2 finale and Orbis.

On a related note, I listened to series 4 finale To The Death last night. Still one of the darkest, horrific, depressing and all-round best finales I've listened to. Like Journey's End with all the worst stuff taken out and replaced with the best of Torchwood: Children of Earth.