Go on, start at the very beginning and work all the way through...
Failing that, the episodes are two per video, in order - and the thing about this series is that by and large you have to watch it in order to really understnad what's going on. This is why it's difficult to recommend individual episodes - as Barbara says, Orbit is a stunner of an episode, but only if you've seen a good proportion of the earlier episodes. As for Blake... well, _don't_ watch it until you've watched most of the series, and don't let anyone tell you the plot.
You can find a set of useful info on the episodes at http://www.hermit.org/Blakes7/Episodes/index.html
including a short (one page) synopsis of the series background, character profiles, and episode transcripts.