Star Trek: Deep Space Nine.
All seven series and 176 episodes finished and very much enjoyed. I can't recall any terrible episodes at all and it worked very well having the final ten episodes as one long running storyline rather than any stand alone stories.
All the characters were beautifully written and there were some truly great character driven pieces among the usual space opera stuff in this case the war between the Federation (Earth, Klingons, Bajorans) and the Dominion (The Founders, Cardassians. It also helped that story arcs continued throughout the series allowing repercussions of past episodes to influence future events and forcing characters to learn that actions have consequences.This trend was especially noticeable toward the series finale, by which time the show was intentionally scripted as a serial
It also improved in it's third series when the space station was given it's own war ship - The Defiant - which meant an end to stories constantly being set on the station. The big action sequences were excellently done showcasing some tremendous space battles that rivaled anything on screen at the time.
Comfortably the best
Star Trek series to date. Recommended.