My biggest musical regret? Motley Crue. I was 14 when Shout at the Devil came out. I bought it and looped it in the cassette player of my '68 VW Bug, over and over, until a much cooler friend of mine noted how ridiculous Motley Crue was, and how ridiculous I was being for letting the band into my personal music collection. Twenty-five years later, I'm still ashamed.
(Advisory to the kids: Some adult language in this video.)
What's your biggest musical regret?