I know this is going to be a flame-war, ridiculous anti-Dak thread, but I will get in my two cents before it gets too crazy.
First off, besides maybe Green Bay we have had the most luck as any franchise with two Hall-of-Famers, and several very solid QBs, including both Romo and Dak. I give props to those two for keeping us competitive through some very below-average staffs and average teams surrounding them.
Of the three, obviously, you would start Aikman, but - I know this will shock some of you - football is a team game, and some of the teams around Romo and Dak were not as good as the teams around Aikman, so I am not sure if he would have excelled as much as he did if had to play with the 2000s Cowboys. If you look at later in his career, his numbers and winning did drop off as the overall talent of the team and the coaching staff dropped off.
I think I would bench late-in-his-career Romo simply because he was very much getting the game figured out, just as his body was betraying him. But, this is not as cut-and-dry as many on here believe, as far as picking him over Dak. I remember many, many games when Romo would pull a Romo pulling defeat from the jaws of victory. With Romo, I felt like we were always capable of beating any team, but we were also capable of losing to any team. And, as has been mentioned, Dak has been a better playoff QB than Romo. So it's tight. I still think, and I am hoping that Dak can overcome Romo.