There are tons of great answers but I believe it comes down to 2 guys; John Niland and Jethro Pugh.
John Niland played for 9 seasons with the Cowboys at LG and was named ALL-PRO 3 times along with 6 Pro Bowl appearances in addition he
is arguably the best Pulling Guard in Cowboys history.
Jethro Pugh played 14 years with the Cowboys. At LDT he lead the Cowboys in sacks 6 times. In addition he was a Brilliant run defender.
He was criminally overlooked when it came to Post Season Honors. From 1967 to 1972 he had seasons of 9, 15.5, 13, 13.5, 13.5, 8.5 sacks and he was 2nd team ALL-PRO in 1968 but was never chosen for a Pro Bowl.
I think even though both are way underrated, I think Jethro Pugh is the choice.