Wacha/Miller/Martinez of the Cardinals
Syndergaard/Wheeler/Harvey of the Mets
Each rotation also has additional young, good pitchers to grab (like Montero, Gonzales, Cooney, etc, but these are the three best)
Even as a Cards fan, I possibly lean toward the Mets young rotation, even though it's a little older, because Harvey could be a constant top 5 pitcher in baseball, and Syndergaard has TORP written all over him. Wheeler is at least a MORP
Wacha I think is a TORP, Miller is a MORP, and Martinez is either a TORP or a future closer. So I would probably take the Mets young three, but I also think it's very close.
I'd say in their peaks...you have the following
Harvey - 6-8 WAR
Syndergaard - 4-6 WAR
Wheeler - 3-5 WAR
Wacha - 4-6 WAR
Martinez - 4-6 WAR
Miller - 3-5 WAR
The issue is Martinez is such a wild card. He could literally be Pedro Martinez good, or he could be...well worthless.
I'd say Miller is a little better than Wheeler. Wacha is slightly better than Syndergaard (same values, but Wacha is MLB ready, Noah needs another year) and Harvey is clearly better than Martinez, with potential sitting out there.
Which three would you rather have to build around?