Dieon didn't play long enough to be considered a best ever Cowboy. Besides, the dude was allergic to tackling.
Bob Lilly didn't just set the tone for the D-line, he set the tone for the entire franchise. Their first ever draft pick anchored the Doomsday Defense and entered the Hall of Fame. Opponent offenses stopped running the ball on his side despite triple teams.
Cornell Green- never played a down of college football, was a basketball player. Made 1st team All Pro and Pro Bowls as a CB then switched to SS and continued to make Pro bowls.
OLB Chuck Howley, SB MVP but easily could have also been SB 6 MVP.
DE Harvey " Too Mean" Martin, was a very pleasant surprise as a 3rd round pick. If sacks were recorded statistically back then Harvey would be ranked 1st or 2nd in NFL history.
Randy White was the best Cowboy defender in history. He playes DT in college but the Cowboys thought he was too small to play the position in the NFL. They wantes to utilize his speed as a MLB.
They were wrong. Once he went back to the D-line he was an instant star, typicaly drawing double and triple teams. If he didnt beat you up bad enough during the game then he would wait for you in the parking lot after the game and finish the job.
The greatest D-line in history was Doomsday Ii composed of three top 5 overall picks, White, Dutton, and Jones plus Harvey.
How good?
In Super Bowl 12, the Bronco's starting quarterback recorded a 0 passer rating. Martin and White were Co-MVP's, they couldn't make up their mind.
MLB Leroy Jordan, best MLB in Cowboy history, once had three INT's in the first half of a game. However, his specialty was stopping the run.
For the decade of 1967 to 1977 the Dallas Ciwbiys were ranked #1 overall in stopping the run.
Despite his problems Hollywood Henderson was probably the best ever Cowboy LB. He was Lawrence Taylor before there ever was a Lawrence Tayor. He used to return kickoffs ran one back for a TD.
Best defensive rookie performance belongs to Everson Walls, 11 regular season INT's for the undrafted free agent back when the draft went 12 rounds. Novody remebers that he had 2 INT's and a fumble recovery in the NFC championship game that year against the 49ers.
Special mention to the greatest defensive mind in NFL history Tom Landry, a former 1st team All Pro CB. He modernized NFL defenses when he invented the 4-3.