I think before sabermetrics, Rickey had a case. But over the last 15 years he has really started to get his due as one of the greatest baseball players of all-time and top 10 hitters post integration.
Musial is tough because he started his career before WWII. He's an icon. I think he's just too old to get brought up in discussions and passed away 13 years ago.
I actually think the most underrated player post-integration is Frank Robinson. Everyone remembers him as the first black manager in MLB and then the grumpy old manager before the Expos moved. He's maybe the best RH hitter since integration behind Mays and Aaron. He also played his prime in the 60's when offense cratered in baseball. Unlike Mays, he didn't have the dazzling defense/SB numbers. Unlike Aaron, he didn't quite have the astounding longevity numbers. But he's a career 153 wRC+. And because he split his prime in 2 cities that are not major markets, he rarely gets mentioned.