Me:
I think Star Trek: the Next Generation; Babylon 5; and Firefly show us some of the best sci-fi on TV.
Conversely, I think some of the worst include some of the more recent episodes of Dr Who (I think Stepehen Moffat needs to step aside and let some new blood in); Andromeda, which always struck me as weird and campy in a bad way...I'm sure I'll think of a tonne more but I prefer positive, quality stories.
I think the three best episodes of TNG are "Family," the episode dealing with the aftermath of the Borg attack in "The Best of Both Worlds," which actually showed heartache and the repair of relationships; "Sarek," the final appearance of Spock's Dad a century after his introduction, who has to pull himself together with Picard's help to arbitrate one last mission; and "Yesterday's Enterprise" giving Tasha the good-bye she deserved and proving TNG had (mostly) left camp behind and stood as quality, thought-provoking entertainment.