Behind TNG, certainly. But TOS? I'd love to see the actual ratings numbers for the original run of TOS, even if they're not as detailed as today's numbers. I've seen the chart comparing the ratings of the spin-offs, but no actual numbers for TOS other than recent re-runs. Are they available somewhere?
I'm sure someone will correct me if I am substantially wrong, but I think it got somewhere around 27 million viewers or so. Remember most folks only got the Big 3 networks (if that) so the national ratings pie was only sliced 3 ways).
"Inside Star Trek: The real Story
" by Herb Solow & Robert Justman gives the info. It is a great read with a lot insight into the production of TOS. From the initial pitch to cancellation. It also gives a more balanced perspective of NBC, Desilu and Roddenberry. Very enlightening. Highly recommend.