Rolling Stone just published a list of the 50 best science fiction shows of all time, and while I loved season 1 of The Mandalorian, I think its presence at number FIVE is a signal that there's a whole lot of recency bias goin' on. Did everything on TV just suck before 2015? No, I don't think so, and I won't even get into shows that could/should be on the list, but the presence of so many series that are so new surprises me (click here for it, but beware, they have giant ads and it crashed my browser). One of them has been on the air only a few weeks!
Doctor Who tops the BOTNS-era shows at #4, though that includes the modern stuff, so maybe that means the highest-ranking BOTNS show is Star Trek: The Next Generation, which ended in the 1990s. Similarly, Quantum Leap started in 1989 and went into the next decade. So you could make the argument that the top "pure" BOTNS-era show on the list is Space:1999 at #31.