For me, it's Bob Seger. I hate myself for how much I love Hollywood Nights. Also, I feel like Seger was probably just another one of those barely talented white boys who made it big because he literally stole a ton of stuff (melodies, basslines, hell even lyrics) from truly talented Jazz, R&B, and Blues musicians...but maybe I'm wrong. Just a gut feeling
Jimi Hendrix, Creedence Clearwater Revival, and Led Zeppelin. Black Sabbath if we're not putting them in the "underground/counter-culture and therefore not really boomer music" bucket.
I would definitely consider Black Sabbath as mainstream, even at the time. I don't think they were any edgier than Zeppelin.
If anyone ever tells you to apologize for Hendrix, CCR, or Zeppelin you can cut that person out of your life. You don't need that kind of negativity.