This is the kind of question that is so carefully formulated that you have few like, reasonable answers. The Twilight Zone is pretty much the only obvious, credible alternative, but past that you're left with arguing the merits of the original Battlestar Galactica or Lost In Space, and while those shows no doubt have fans who may argue they were better than TOS, how many of them post on a TOS subforum of a Star Trek website?
TOS is the best American sci-fi series until 1985 (which is a date I just plucked from thin air)
Totally. That Star Trek: The Next Generation
aired the next year and a plethora of well liked sci-fi programs also came into being thereafter is pure happenstance.
Well, following Sturgeon's law, the vast majority of television sf would be crap. Still, I think the output during the 60s and 90s was far more interesting than that of either the 70s or the 80s (excluding TNG, although I think the best of that series was produced in the 90s).
I would absolutely agree with this. They're easily the two best decades of sci-fi TV (the 90s in particular - simply because of the quantity
of quality sci-fi series, not that those series were better than the cream of the 60's crop).