Some SciFi choices:
Babylon 5 - Hasn't aged the best, since it was one of the first shows to use CGI regularly, and it shows. They also lost the film in a fire so they can't remaster it. It has some of the best character development I've seen on TV though.
Stargate - 17 seasons across 3 series is not an easy feat. Again, takes a little work to get into. Acting in the first season is stale. Once it gets going though, you can't stop.
And of course there's Star Trek, but what no one tells you is that Deep Space 9 is the best one.