Babylon 5 *
Buffy
Angel
Futurama
Star Trek: TNG (and TOS, though I wasn't around for it)
Quantum Leap
Dark Angel
Doctor Who
...are we detecting a theme here??
* Babylon 5 was planned as a 5-year story arc, so it's not surprising that it went off the air after 5 seasons. The trouble came when the network started talking about cancelling the show after only 4 seasons. The creator, J. Michael Straczynski, had to scramble with rewrites to try to tell the final two years of the story in one season... then the network turned around and renewed B5, so JMS had to come up with some story to tell during the resurrected fifth season, and, well... it wasn't the best season of the show... not surprisingly.