This week's most underrated philosopher of the week has the sixth hightest cited paper in the history of Erkenntnis (ahead of famous papers by Carnap, Neurath, Kuhn, Davidson, Putnam, etc). He has a paper that is among the 30 most cited pieces to appear in Philosophy of Science (even though it appeared only in 2002). Surely, this is partly the product of the recent popularity of philosophy of biology and the significance of mechanism(s) in discussion of causation. But Stuart Glennan (who teaches at Butler University) also deserves credit for writing about mechanism when it wasn't hot yet.
But Glennan isn't just a narrow philosopher of biology. (The rules for 'most underrated' require 'ínteresting' breadth in addition to excellence.) He has also written on science and religion (not uncommon for philosophers of biology and not surprising given that he teaches in a combined philosophy and religion department). But he also publishes influentual work in philosophy of education. Citation metrics don't tell the whole story of philosophy. But sometimes they can point us to somebody who is clearly most underrated!