Oh, this is terribly terribly hard, I couldn’t possibly only have one answer so I’ll pick one from each series.
In Series 1, I love Ghost Town because lets face it, Gilbert was amazing. In Series 2, I really like Damage because I love Daisy. Series 3 would have to be The Wolf Shaped Bullet in honour of Mitchell. And In Series 4 I loved the Graveyard Shift because I found it hilarious.