I got into a big discussion today with a friend of mine as to why I thought DS9 was the natural heir to TOS. It did all of the things I loved so well about TOS, but with less crack and more thought. There's a reason, I think, that the furthering of TOS character stories happened in TNG (Unification, Relics), but the sort of meta commentary on the value of TOS happened in DS9 (Trials and Tribblations, Far Beyond the Stars). DS9 took that moral component of TOS and really questioned the rightness and wrongess of the edifices of the ST Universe itself.

Don't get me wrong, I enjoyed the other series. But DS9 captured my mind, and I think that's why I appreciate it so much at the end of the day.