I think that Ken Singleton is one of the most underrated players of his era. He played for 15 seasons, and posted an OPS+ of less than 100 only twice: his rookie year (when he was right on the cusp with a 99 in 76 games), and his final season. Overall, his OPS+ was a very good 130. Ken had that valuable combination of on-base skills and power that made him a highly productive player: he had five seasons of 20-plus homers and another seven in double digits, as well as an impressive .388 on-base percentage. He routinely carried an on-base percentage that was more than 100 points above his batting average, walking 90-120 times with regularity. For his career, he walked slightly more than he struck out. But you don't often hear Singleton discussed as one of the best hitters of his day.
I recently pulled out one of his cards and looked at the stats on the back. I knew that Ken must have been underestimated even when active, since the Mets and the Expos both let him get away in the early years. But I was still amazed to see that in 1973, as a 26-year-old fourth-year player, the outfielder hit .302 with 23 homers, 103 RBI, and 123 walks (that's a .425 OBP!) for Montreal. Sure, his power dipped the following year (20 2B, 9 HR), but he still got on base at a .385 clip in a "down year". How could the Expos turn around and trade him entering his prime? And how could they also give up on a solid 27-year-old starter like Mike Torrez? What could be worth that? Surely not a past-his-prime Dave McNally and a struggling Rich Coggins. Our gain, I guess.