Counting only players from our base 4-3 defense (so not including Trufant, Johnson, Smith, etc), I'd say Leroy Hill. Hill has pretty much been the definition of average this season.
Chancellor is probably the 2nd "worst." He's still a good player, but this year he's more pedestrian whereas last year he was making plays everywhere. It could just be that our defense is so good now that he has less opportunities to make impact plays.
Jason Jones is the 3rd "worst" if you count him as a starter. He's not bad against the run, but so far he's looked like our 3rd best pass rusher at DT. Both Mebane and Branch have created more pressure. In my own observations, at least. Jones is an okay player, but I wouldn't bring him back for another $5 million if I was the GM. I'd definitely bring back Branch though, he can back up Mebane and play 3 tech too. I didn't think it was possible to have a more under-rated season coming off an under-rated season, but Branch is doing just that.