In compiling a list of underrated players, I find myself coming up with a lot of players from the 1970s and '80s. This might be a product of my natural bias I was a young fan in those years, and we tend to glorify the heroes of our youth but I don't think so. Rather, I think it's because there were more teams and more players in the '70s and '80s than there had been, so it was easier for great players to slip between the cracks. Also, the Hall of Fame has done a pretty good job of electing marginal Hall of Fame talent from the old days, but has (somewhat intentionally) resisted the temptation with more modern players. And it's hard to be underrated if you have a shiny plaque in Cooperstown.
Catcher: Ted Simmons (3.74)
That number in parentheses? That's the highest percentage of Hall of Fame votes Simmons garnered; it takes 75 percent for election. Simmons got 17 votes (3.74 percent of the electorate) in 1994, his first year of eligibility and also his last, as he fell off the ballot for lack of support. But just for a moment, wipe the name "Ted Simmons" from your mind and let me describe a nameless player: "decent defensive catcher, eight-time All-Star with 2,472 career hits, No. 1 among players who spent most of their careers behind the plate." Sound like a Hall of Famer to you? I haven't even mentioned nameless player's power; his 778 extra-base hits rank behind only Carlton Fisk among catchers.
As I suspect you've suspected, this player does have a name, and it's Ted Simmons. Oh, I know people think Simmons was an awful catcher, but for the first half of his career, he was decent enough. He wasn't Johnny Bench (nobody was), and people probably held that against him. And Simmons' career didn't end on a happy note; he spent his last five seasons as a weak-hitting DH/PH type, and people probably held that against him, too. But Simmons was a great player, and all the Hall of Fame voters in North America won't convince me otherwise.