Stephon Gilmore
The best free agent cornerback that is still available is Stephon Gilmore however there are many issues that make a reunion in Western New York unlikely. The Bills drafted Gilmore in the first round of the 2012 NFL Draft and he played five seasons in Buffalo.
He was a starter almost the entire time with the Bills and would earn his first Pro Bowl nomination in his last season with Buffalo. However, the Bills were not able to re-sign him and Gilmore would go on to play four seasons with the Patriots and recently has had one-year stops with the Panthers, Colts and Cowboys.
The challenge for the Bills, aside from Gilmore's personal opinion of Buffalo, is that he has primarily been a starter everywhere he has played. It might be a tough sell for the Bills to Gilmore that he would be coming in as a backup, or have to earn that starting job.
While at one time Gilmore was one of the best cornerbacks in the NFL, at this point of his career he likely isn't an improvement over Benford.