Love Open Range. "Men are gonna get killed here today, and I'm gonna kill 'em."
I'll agree that Open Range is an underrated movie, and Kevin Costner was great in it. He got his due back in the day, though, then kind of disappeared - Waterworld really sunk his career, as I recall it.
IMO, when Hollywood keeps giving you starring or major role in huge movies (Brad Pitt, Tom Cruise), it takes away any claim to "underrated" you might have had. Tom Cruise is a great example of someone who's been overrated that way - constantly a star of movies in which dozens of other actors would have seemed better choices.
I can agree he's had some great roles that might have been underrated (Born on the Fourth of July was a great movie, great performance), and maybe underrated by critics (wtf cares what critics say), but no way has someone with a career like that, complete with all kinds of awards, been underrated by their peers or the industry.