I just posted a message on another thread about playing games. This seems like a good game, so I'll start...
I'm a science fiction fan, but that's not the whole reason for my #1 choice:
Stargate SG1
In my opinion, this show wins the best TV series of all time. They did ten seasons, with 20+ episodes per season. The first two seasons were just kind of action with a thin plot, but after that they started making more money. They got better writers, the actors settled into their roles, there was good character development. Best of all, the stories started getting really good. The framework made for a limitless variety of plotlines.
I highly recommend starting at episode one, and watching them all in order.