There are debates over, say, the Baseball Hall of Fame, but with sports there are objective statistics to measure greatness that fans and writers can use to make their case for this player or that coach.
Setting up a Hall of Fame for music is trickier because no one can even agree on what metrics should be considered to determine who makes it in. Is it commercial success? Critcal acclaim? Influence? Longevity? Some combination of those?
Popular art is too subjective for there to be a Hall of Fame that everybody (or anybody) feels represents most or all the important contributors.
But as long as the Rock and Roll Hall of Fame exists people will be lobbying for their favorites to be included. With that in mind, here are some artists with a more impressive body of work than Cat Stevens, whether the Hall recognizes them or (probably) not.
The '5' Royales: The 1950s vocal group began the decade playing raunchy R & B like "Baby Don't Do It" and "Laundromat Blues" and ended it with the dance stomper "The Slummer the Slum" and the original "Dedicated to the One I Love." They were led by singer Lowman Pauling, maybe the most underrated guitarist in rock history, and one who was revered by Steve Cropper.
Roxy Music: The band's 1972 debut was abrasive and arty, but after Brian Eno departed Bryan Ferry took over its increasingly slinky, sensuous sound. A more polished version of that sound could be heard in early-80s new wave groups like ABC and Duran Duran, but Roxy outshone them on its own 1982 Avalon.
Sonic Youth: The 1988 album Daydream Nation is an alt-rock landmark, but New York's Sonic Youth continued to record brilliant albums full of exquisite guitar over the next two decades. Vocalist-guitarist Kim Gordon has become one of the most respected female musicians of the last 30 years, though her recent messy breakup from co-leader Thurston Moore means the group is likely finished.