To this day, I don't think Star Trek: The Original Series and The Twilight Zone have been topped.
To me only X-files comes close, but saying that makes me really feel like I'm just being biased against other good sc-fi.
Were they really that good or is a huge part of it nostalgia?
The seventies and early eighties were a dark time. But, as others have already pointed out, there's been plenty of good stuff since: the Trek spinoffs, Farscape, Firefly, etc.
And there's even more if you broaden the net to include stuff like Buffy, Xena, Warehouse 13, etc.