Has to be The X-Files. One of my absolute favourite shows ever to air, at least for the first 6 seasons. I have marathoned the whole show three times in the past decade and appreciate it more each time. Favourite episodes include Bad Blood, Home, The Post-Modern Prometheus and Jose Chung's From Outer Space.
Most fans will say it turns to shit after Mulder leaves but I actually think Season 8 was better than the very tired Season 7. Season 9 is shitty though and I Want To Believe is a bizarre mess. I have no iea what Chris Carter was going for there. It's a poor Silence of the Lambs knock-off that arrived way too late to seem interesting.