This has been really hard for me to recall! Any R movies I saw as a teen were on cable at a friend’s house, as the Lord intended.
Our old-school family-owned rental store not only carried Angel and its sequels, they kind of promoted the movie. Life-size cardboard cutout display of Angel plus a few posters scattered around the store. Perhaps the proprietor was a big fan?
It was in 1978, and it was one of these three: Saturday Night Fever, Rocky Horror Picture Show, or Kentucky Fried Movie. I forget which order I saw them in and only Saturday Night Fever was in its first run. I could have sworn that Bobby Deerfield and Wizards were both R at the time, but Google tells me otherwise. A lot of movies have dropped down a rating since their original release. If I recall correctly, 1979 was the first year that the R rating was sought out by filmmakers because it was actually more lucrative.
Probably Revenge of the Nerds. My friend and I, both about 12-13, rode our bikes from our homes in Rochester (suburb of Springfield to the southeast) to the movie theater on the southeast end of Springfield, about 6.8 miles away per Google Maps. We bought tickets to Ghostbusters and instead went into the room where Revenge of the Nerds was showing. We thought we were being so slick, but undoubtedly the teenager working the ticket counter knew exactly what we were doing and didn’t give a shit.
Stripes
That I can reliably identify in my memory as to what and when, Dog Day Afternoon.
Mom & Dad took me to The Blues Brothers on the advice of my brother, the Army vet.
I agree, at least regarding The Blues Brothers; the home video version was re-rated as PG-13.
Probably Rocky Horror Picture Show at a midnight showing. I’m going to take a wild stab and guess that I was about 15 or 16 and went with my older sister. The usual raucous crowd participation ensued (although not to an extreme level). It was fun but not remotely scandalous.
It does amuse me that RHPS now streams on Disney.
Cocaine and atmospheric lead poisoning explains so many questions regarding the early eighties.
My father took me to see Monty Python’s Meaning of Liff LIfe and Excalibur in the Theater. I’d already been sneaking rated R movies on HBO for a while at that point.
Wasn’t it later recut and released as a PG version?
Yes, it was. Two years later.
Looks like it, and the edited version apparently came out soon after the original. As per Wikipedia:
That varied by theater. I distinctly remember taking a 16-year old girl to an R-rated movie at a neighborhood theater. The cashier ID’d us, and refused to sell her a ticket. He didn’t consider 17-year old me to be a “guardian.”
In 1979
The PG version of Saturday Night Fever was the only one I knew for decades until I happened upon the original cut a few years ago. An entirely different (and ugly) motion picture.
My mom didn’t think it was a kid’s movie, she thought I was mature enough for a grownup movie. I remember taking it as a compliment.
Maybe The Exorcist where a lot of people walked out.
I gathered that from the context, but used it to make a more general observation. Certainly a title like that is going to make most anyone not in the know think it is G-level kid fare (and I certainly did having only a vague recall of said flick’s title until you clued me that it apparently is not).
Fist of Fury, although back then, it was released in the U.S. under the title The Chinese Connection.
I was 5 years old when it was released in 1972. My family saw it at a drive-in, not a regular theater, so I might have been 6 or 7. My brother, a dozen years older than me, was a big martial arts fan. He probably persuaded my parents to take us.
Until I saw it on TV recently, the only thing i remembered from the film was the exotic dancer. When her scene started, suddenly my mother insisted that we all make a trip to the snack bar. Alas, her attempt at distraction failed. 
Ten is not a scary movie. But it is a sexy movie, and while i might have been okay seeing it with just my parents, going with my little sister and my great aunt was deeply uncomfortable.