Please advise best airline Washington,DC - Seoul economy class roundtrip. Thank you.

You might try ANA from Washington to Tokyo and then Asiana from Tokyo to Seoul. Check out www.kayak.com to see if the connections work. There are no direct flights. Note that some flights are code share so you could get stuck on United if you don't pay close attention.

Korean Airlines has nonstop flights several (4) times a week, out of Dulles Airport. I've flown them several times and it is quite a nice flight with the only problem is they tend to keep the cabin a bit hotter than most people seem to like. Good food and service.

Also, Asiana has direct flights but there's a stopover in Los Angeles which is about 4 hours.

If you're required to take a US airline, Delta's daily flight out of Atlanta is not bad at all.....

Thank you - I will follow up on your suggestions

Second to the advice of Korean airlines. We've flown to Vietnam and China with them and have never been disappointed.

In addition, my husband swears they employ the most beautiful women he's ever seen!

You might want to review the accident rate for Korean Airlines. Here's a resource to get you started:

http://www.airsafe.com/events/regions/asia.htm

Delta dropped Korean as a partner for a while after 1999 (prompted by the Shanghai crash) because of its safety record.

They've been making serious efforts to improve:
http://koreabeat.com/?p=1943

But personally, I'd choose ANA + Asiana, or United.