1. Beach Boys' non-surf music, post 1965, including Smile and albums through 1971-1972. 2. Nick Drake. 3. Velvet Underground (may be well-known to serious rock and rollers but still undervalued among the general public). 4. Forever Changes.
As for Nick Drake, I've seen many stories/articles about him in various rock magazines over the years. That's not keeping a secret.
Unless one is a hardcore Aerosmith fan, it's not very well known that Joe Perry and Brad Whitford were not the ones responsible for that stellar guitar playing on the Get Your Wings version of "Train Kept A-Rollin'". It sure was a letdown for me when I first found out. I still feel disappointed about it to this very day.
The Velvets are not universally recognized as among the most influential bands among day-to-day rock fans. Same goes for Drake. We all know the Velvets and Drake well, but most people can name maybe one or two VU songs and nothing by Drake, if they have even heard of him.
None of these are secrets, that is well-known music to anyone with more than a passing interest in rock of that era, although at the time of release some of that was pretty close to secret. That Bob Dylan escaped being drafted and sent to Vietnam in exchange for disavowing politics...now THAT is a secret. It isn't true, of course, but if it were it would qualify.
Bill Nelson/Be Bop Deluxe - especially in America I didn't know that. Who played guitar on on it then?
Nick Drake songs have been in VW and AT&T commercials. I'd agree with the Beach Boys. I'd add Del Shannon outside of Runaway and the other early hits as well as Charlie Rich pre-Billy Sherrill
I think you guys are being too rough on the OP and overestimating the general public. And someone being in the Rock and Roll Hall of Fame doesn't really have anything to do with the general public knowing who they are, and especially being familiar with their music. Just because someone has heard a song in a commercial doesn't mean they know who it is or are remotely familiar with their music. Edit: That being said, at least with my peers when it came out, Nick Drake saw a pretty big boom of interest due to being featured in the Garden State soundtrack. I think that says a lot more about the people that really dug that movie as opposed to the general public though.