A lot of the plays mentioned above are somewhat symbolic (Bob Lilly's 29 yard sack of Bob Griese), and came after the Cowboys were established as one of the league's titans (Hail Mary). The Dorsett 99 yard run came in a game that the Cowboys lost, so that can't qualify.
The Harper slant play was big, but the Cowboys were in the lead at the time of the play.
I'll go off the beaten path here, and select a play that many wouldn't even consider.
In 1968, the Cowboys played the Browns in the Divisional Round of the playoffs. Despite the Cowboys having the better record, the game was played in Cleveland. The field was a muddy mess.
The Cowboys got off to an early lead, but the Browns rallied to tie it at halftime. Early in the third quarter, Don Meredith threw a pass that intercepted by Dale Lindsey, and was returned for a touchdown. It gave the Browns a lead that they wouldn't lose. It also turned out to be the last down of football that Don Meredith ever played. Landry benched him for Craig Morton, and Meredith retired before training camp in 1969.
Why would this be the most important play? If Meredith doesn't retire, Roger Staubach, just back from his tour in Vietnam, likely doesn't make the team. Without Staubach, the Cowboys don't become THE Cowboys.
If I were to pick a seminal event, it would be in 1965, when, following a horrendous game by the Cowboys and Meredith against the Steelers, the team was under siege by the media. Clint Murchison had given Landry a 10 year extension, and the team appeared to be stuck in the mud. It was here where Gary Cartwright wrote the article channeling Grantland Rice, referring to the Four Horsemen as Famine, Pestilence, War and Meredith.
In practice following this loss, Landry broke down crying in front of the team. From that point forward, the Cowboys fought to finish with their first non-losing season, and didn't have another losing season for 20 years.
