Most underappreciated/undercollected players

[nat]

The answer is probably, like someone said upthread, a great player who isn't in the hall (yet). Or maybe a great player who played between the tobacco era and the gum era.

Under the former heading I'll nominate Bill Dahlen and Lou Whitaker. (Although Whitaker playing from the 70s to the early 90s really limits his collectability in terms of baseball cards anyone might actually want.) Under the latter heading I'll second (or third?) the nomination of Grover Cleveland Alexander. He's basically a half step ahead of Matty in terms of career production,* but not held in nearly as high regard by collectors.

*
Pete: 135 ERA+, 5190 innings, 120 WAR
Matty: 136 ERA+, 4788 innings, 104 WAR
That's a dead heat in performance and a small edge to Alexander in durability.

(I imagine that the fact that Pete's WAR advantage is larger than his IP advantage is due to better defensive players on the Giants, but I'm not going to dig into the calculations to verify that.)