Not necessarily a great fighter but one who's supporters were loyal, passionate and convinced of their greatness. Could be because of his style, achievement's or personality . Who gets your vote? Let's say fighters before 1990?
Larry Holmes. You'd swear he never lost a round, got knocked down, or struggled with supposedly inferior fighters. He gets defended on here like he's barely a shade under Ali's greatness at the GOAT. It's delusional, really.
I dont know if it was fanatical support but if you are talking about being loved then look no further than Frank Bruno
Duran's fans were pretty fanatical. He was so popular at one point that the Panamanian government said he no longer had to pay taxes. Then he quit against Leonard, and the venom of those fans got dangerous. (Of course, the government rescinded its no-taxes gift, too.) Sometimes you don't know how fanatical fans are until the fighter disappoints them.
He had a hell of lot of support mate. Not sure even Henry Cooper had that? Could be wrong, he's a bit before my time is old Enry
Mike Tyson (well, you said the criteria was pre-1990). He has fanatical fans that weren't even alive when Tyson was champion and undefeated (until the age of 23, that is).
Duran and Holmes were mentioned. Marciano, Dempsey, and Tyson. All very exciting fighters to watch whose fans get a bit carried away.
Julio César Chávez more than any heavyweight champ. May have been other Hispanic fighter greater popularity but not sure however Chavez was a god to Mexicans and other Hispanics.