Battlestar Galactica
Probably my all time favourite Sci-fi show and right up there with Breaking Bad and Sons of Anarchy with my top 3 of all time. The show is just perfect, the casting is realistic, the story-lines are immense with some great exciting moments. It is full on thoughtful science fiction with enough "Pew Pew" moments to raise the hairs on the back of your neck. The progression of the show and its characters over all the series works really well with the story-lines building and never going too far away from the general feeling of humanities threatened existence. I'm not sure this show will ever be bettered...