The 90's Cowboys were the most dominant team of that era and at the time the greatest dynasty of any era. .
Depends on how you define "dynasty."
The 1960-1967 Green Bay Packers won five titles in eight years and a roster with 10 Hall of Famers on it.
1982-1994 San Francisco won three Super Bowls under Walsh before he retired following the 1988 title. George Seifert took over, winning a Super Bowl with Young in 1989 and 1994. During this 14-year stretch, the 49ers won five Super Bowls, lost it to the NFC Championship Game four times and posted a 159-56-1 record.
From 1970-1979, the Cowboys had a 105-39 record in the regular season, four Super Bowl appearances and won two rings.
Between 1972 - 1979 Pittsburgh cast of Hall of Famers, and legendary coach, won four Super Bowls across nine seasons.
No doubt they were the best of the '90s, but you'd have a hard time proving they were the best up to that time.