Not long ago, hitting 500 home runs was big news. Before 1990, only 14 hitters had hit 500 home runs, and all of them were in the Hall of Fame (or would be soon). Since 1990, six other hitters have reached 500, and three or four more likely will do it this season. So when asked to name the most underrated hitter with at least 500 career homers, one is tempted to answer, "All of them before 1990."
That's not much of an answer, though. Anyway, it's hard to argue that Hank Aaron (755 homers) or Willie Mays (660) or Mickey Mantle (536) is underrated. Or Babe Ruth. Frank Robinson, with 586 homers? Sure, he's underrated. He's one of the dozen or so greatest outfielders ever, but he was just the third-greatest outfielder during his career, which left him underappreciated in some quarters. Still, Robinson did win MVP awards in both leagues, so it's not as though nobody noticed him.
Mel Ott, who hit 511 homers during his long career with the New York Giants, is another candidate, and I suspect far too many baseball fans today have just a passing familiarity with that name. At best. But Ott certainly was appreciated during his career and slid right into the Hall of Fame the moment he was eligible.