Judging by your location, your best bet would be either with AA or US/UA.

I would suggest AA. They offer more room throughout coach for all coach passengers (except 757 and A300 which are for leisure routes). They have a very strong presence throughout the east coast, along with US and UA (in IAD).

With AA, they determine your elite status by either Q-Miles or Q-Points, whichever one gets to the elite threshold first. For example, if you are a Gold flying full fare economy, you will get 100% Q-Points + 25% GLD rewarded Q-Points. You will receive 100% Q-Miles. For discounted economy, as a gold, you will get 50% Q-Points + 25% GLD Q-Points = 75% Q-Points. You would get 100% Q-Miles.

Basically, AA doesn't punish you if you fly discounted economy as you will get 100% Q-Miles either way, it only helps you when you fly full fare economy and are an elite member.

If that confused you, be rest assured that you will get 100% elite-qualifying miles for any economy fare, including discounted economy.

As for the AA network, they have tons of east coast-west coast flights. They also have a very strong network to basically anywhere you want to go through their hubs in Chicago, Miami, Dallas, San Juan, and St. Louis. They provide east coast shuttle flights with American Eagle.