I think the 3x 7 episode stories in S7 were designed to save money, but they are a bit padded out, Ambassadors of Death being the one that creaks the most. Inferno explores some interesting territory and I particularly like the ending of episode 6 (?) where the parallel earth comes to an end and they all realised that they're b@ggered.

I wholeheartedly endorse the Green Death as being the next one that you should watch. It scared the life out of me as a kid and on the whole has stood the test of time. Beware of some dodgy visual effects though- I won't spoilerise it for you, but you'll see what I mean..