Blake's 7 is without a doubt one of the best sci fi shows of the 20th century (behind it's better known sibling Doctor Who). For it's time the special effects are top notch, the stories are compelling and engaging, and the characters are believable.
If you like Joss Whedon's Firefly, chances are you'll like this series, with some similar themes including the main theme of outlaws rebelling against an oppressive government.