Assuming that price, miles, perks, etc. don't matter, United into Dulles is best because it's a UA hub, and if there are issues they have more than one way to get you there.

Dulles is my home airport, and I rarely have issues with United that they can't at least route me through. But I've been royally screwed on AA and particularly DL in and out of Dulles.

And always take the early flight to avoid cascading delays.