Most underrated: Scott Brosius
Just a ballplayer and a winner. Grinded it out every at-bat. Sticktoitiveness and moxie off the charts. Was just clutch, in a way that you can't quantify. I don't even know if he played in the last 20 years but he was the first to come to mind. Honorable mentions: Jhonny Peralta, Gary Sheffield.
*Upon further review, I think Blake ranks but isn't 'the most'. There were warts: Casey Blake had one of the ugliest swings I have ever seen, was awful in big situations (just looked like he was guessing) and was a bad 3b while w the Tribe. He deserves mad props for making himself into a bigleaguer after the Twins never gave him a chance. I think fans here definitely underappreciated him at first (largely related to how awful he was with RISP), but by the end had an appropriate appreciation. Esp after he brought back Santana, who still inspires horrible jokes about the guitar player even though they stopped being funny years ago. I think the latter part of his career he was rightly appreciated. Checked off many of the same boxes as Brosius but was never as clutch, which is why he was discounted.