Troy Brown in 2001. He had an awesome year, one of the very best seasons by a Pats player in the Belichick era. It's right up there with all of Brady's great seasons, Ty Law in 2003, Tedy Bruschi in 2004, Corey Dillon in 2004, Richard Seymour in 2003, Asante Samuel in 2006.
It's also premature to compare Welker's season to Brown's anyway, as the season has yet to be completed. Brown had special plays in huge games down the strech.
Brown had three punt return touchdowns in 2001, including a huge one in the AFC championship. Then take into account he had the big play on the game winning SB drive.