UA has the best alliance, and best FFP.

DL offers the best hard and soft product.

AA offers the best blend of FFP and product.

WN offers the best product + FFP for purely domestic passengers.

For most passengers, the biggest airline in town should get their business. In competitive markets (NY, DC, LA, ORD, SF, LV, SEA for now) the difference is more about what brand/staff/etc match travel patterns.