What are the best US airlines to fly? I'm looking for quality in economy seats?

Also visually appealing is another bonus.
And it has to be not too pricy.

There aren't any, they all SUCK big time.

United airlines

The most cost-effective, acceptable one that serves your itinerary.

The majors are best, Delta, American, United, etc. I'd include southwest too as they are the better of the low cost airlines.

I'm a strong Delta supporter. Their seats are super comfortable, well kept, and not ugly. And a plus is the screen with a good selection of movies, music, and tv shows. The people are super nice as well!

You're not going to get that on a US airline. They all try to squeeze in as many seats as possible in economy, so they can show profits to their shareholders. They have consistently decreased service (no free food) and increased costs (charging for baggage). Insiders say that they calculate just how uncomfortable they can make you in economy so you'll pay for business class next time. The only exception is if you're in their frequent flyer programs, where you can sometimes get a bulkhead or exit row seat.

Quality in economy? Look at Emirates or SAS. You won't find it with a US airline.

I prefer Delta for both domestic and international flights.

I have flown on many of the other "premier" international airlines and that includes Emirates, Qatar, and Singapore. Also flown on almost all of the major European airlines. Delta is at least their overall equal in Economy Class, and Delta's video entertainment on long distance flights is superior.