Best route: Air Canada through YYZ.

Second best: Take United through ORD.

Third best: Fly to LAX and connect to US Airways. (Not SFO, because US is in a separate concourse which doesn't have a lounge.)

Don't go through IAD. You will have to take a United Express flight from IAD to PIT. United Express at IAD is the very worst airline experience in the entire Star Alliance (yes, worse than a transcon on US).