I can't recall seeing much of Twilight Zone
but I can accept it as first or second. However after (say) TOS and TZ, I always liked Planet of the Apes.
... As for British Sci-Fi shows from the same era upto 1985, there are plenty to choose from. But really only one has had the staying power. Whilst some have remained in the public mind others have faded away
Captain Scarlet and the Mysterons
The Invisible Man
The Tomorrow People
Sapphire and Steel
The Day of The Triffids
I could go
[... Given that the standard-bearer of TV sf from the 1970s is probably Space: 1999, ... .
I just want to say one thing: AHEM!