8. Gridlock
This is probably the weakest point on my list, because I'm aware that a lot of people loved Gridlock. And I guess at its heart it had a rather brilliant premise concerning urban migration, drug addiction and Ardal O'Hanlon dolled up as a cat. But still, I think one aspect of this episode went tragically underused the mood patches. We're introduced to the mood patches extremely early on when a young woman takes something to help her forget her parents, who have joined the motorway. Their power is made apparent by how quickly she forgets everything about them, leading us to think that perhaps this drug might be of central use to the story. Sure enough, we're right it turns out all the inhabitants of the New New York's upper level have been dosing too often, until they came across something called Bliss, which proceeded to wipe them out in a matter of minutes but not before the Face of Boe closed off the lower levels of the city, leaving it in a state of depression with an auto-running motorway to nowhere. To me, those mood enhancers were a missed opportunity they could've created the sort of mind-bending tale Steven Moffat is famous for, but here they're treated as little more than macguffins to link the beginning to the end. Their potential as a dramatic device was limitless think The Silence's amnesia hook, but add a whole glut of other emotions into the mix. This was something you could've easily used too create something creative, clever and affecting, but unfortunately they dropped it for most of the plot, creating an adventure which while competent, didn't hit the highs it could've, Yana twist be damned.