Every one of Marilyn Chambers' scenes in Insatiable are legendary.
While I preferred Ginger Lynn's looks there's no denying that Marilyn Chambers had phenomenal energy and consistency (did she ever do a bad scene?) She was also lucky enough to star in the best adult movie I've seen.
I'd have to say Behind the Green Door however, Up 'n' Coming and Insatiable are close runner ups.
Edited to add, if you are only looking for films from the 70s only Behind the Green Door (1972) from my list fits as Up 'n' Coming is from 1982 and Insatiable is from 1980. However I wouldn't rule them out based on their release dates.
Insatiable #1 and #2 are both good, as is Up N' Coming. Also check out Marilyn Chambers Private Fantasies #2-#6. I-candy released all of these discs. Their web site is: http://www.icandvd.com I believe there is also 3 different 3 disc box sets available. One of them has Insatiable #1, #2, and Up N' Coming.
Insatiable. Favorite scene is when she picks up the guy who was out of gas, then pulls over to blow him while he leans against her sports car. Not too many movies from that time have complete start-to-finish blowjobs like this one. Lost count how many times I've watched this scene.
Insatiable was just a hot film overall! I also repeatedly watched the scene with Jessie St, James and John Leslie, with him telling her to "choke the chicken". It was outside and dark which added more to it I think.
Also that scene with Marilyn blowing the kid by the car kept my horny mind racing all through high school!