For US Airways, i think it would have to be the 737 and A330.
Photo © Mark Kopczak
Photo © Carlos Aleman - SJU Aviation Photography
When I think of Delta, I think of the MD-88. Continental I think of the 757,777
Photo © Jason Bisson
Photo © Andrew Compolo
Photo © Andrew Scott
The DC-9 represents NW. And the 747,777 are United's.
Photo © Dan Brownlee
Photo © Ben Wang
Photo © Justin Idle
Photo © Ralph Duenas - Airplanespotters
Photo © Joe Pries - ATR Team