B leaguer
A B leaguer is a musician or band that's barely missing out on being top-tier. They're like the guy who almost made the team but got left out because he was just a little slower.
"You’re not bad, but you’re not the best. You’re like the second best in a room full of first places.", A fan comparing a B leaguer to an A leaguer
"You’re like a B leaguer. You’re good, but not good enough to be on the cover.", A critic giving a lukewarm review
"You’re not the A, but you’re not the C. You’re the B. The middle child of rock.", A fan giving a B leaguer a nickname