Is there a preferred airline from the western United States to Argentina? Are some providing direct flights?

Your best bet is either American Airlines out of Dallas, or Continental out of Houston.
If you opt for Delta, you'll need to go to the East Coast to get to BA. American also goes from JFK, and I believe a couple other hubs.
tptr

Lan also provides great service.

A little more detail...American flies from MIA as well as JFK and DFW. Delta flies from ATL.

I agree that LAN is an excellent airline.

No direct flights, as far as I know.

As the above posters have told you, the main alternatives are through Houston, Dallas, Miami or Atlanta.

Have a great time in our country.

Yes,
I was not slighting LAN, but from the western US you'd have to get to Los Angeles to fly with them, and then fly to either Chile or Peru before going onward to BA.

That said, if you don't mind all the flight time, a quick look at www.LAN.com and I found fares for under $1k, which seems to be better than US airlines offer.
tptr