Emmitt Smith and Bob Lilly.
Emmitt is arguably the greatest RB of all-time, and Lilly is arguably the greatest DT of all-time.
Roger Staubach was fantastic, but he isn't arguably the greatest QB of all-time. Tom Landry and Tex Schramm are too cheap and easy to pick. Larry Allen is a good argument because he really was dominating, and he's arguably the greatest LG of all-time. But he didn't bring the leadership that the other guys mentioned did, and as a LG you can only impact the final result of the game so much.
In the end, I go with Emmitt and Lilly. Emmitt is the most successful Cowboy of all-time. When you look at stats and winning, no other Cowboy even comes close. And Lilly set the precedent of greatness from the very beginning.
My list would look like this:
1. Emmitt Smith
2. Bob Lilly
3. Roger Staubach
4. Randy White
5. Larry Allen
6. Tony Dorsett
7. Michael Irvin
8. Troy Aikman
9. Mel Renfro (why does everyone always forget about him??)
10. Bob Hayes