My vote is based on how Cowboys fans rate the team's players, and I'll say Roy Williams is the most underrated. Sure, he made the Pro Bowl, but way too many of his own team's fans (as well as a lot of other people) think he's overrated -- but he's not. As I've said many times, he makes more big plays in all aspects of the game (sacks, interceptions, forced fumbles, etc.) than any other safety. Some guys get more interceptions (Ed Reed when healthy) but don't get the sacks or forced fumbles. Some guys get the sacks (Adrian Wilson) but don't get the interceptions or forced fumbles. Roy makes big plays in every different way, along with his big hits and run-stopping ability, and he does it despite not being used in a way that would maximize his impact. He gets knocked for supposedly going for the hit and missing tackles, but that's no more common -- and probably less common -- than the missed tackles by any other Pro Bowl safety. And he gets slammed for his coverage. But having charted every pass attempt against us last season, I don't think his coverage was a problem at all. There were very few plays when I marked him down for allowing a completion of any significance.
Al Johnson and Aaron Glenn might be underrated by more Cowboys fans, but those who underrate Roy do so by a greater degree.