I find that arriving BKK late evening works best for me since I can just go right to bed when I get to the hotel.

From IAD there are lots of options. I think the shortest travel time is probably still via PEK on UA and then connecting to Thai Airways. But the problem with that is you have to fly UA and then connect in PEK.

Another option is to fly via NRT on ANA with options to connect to either ANA or TG in Tokyo. Or you can fly via ICN on Korean.

The last several times I have gone I've used AA miles so for me that means flying via either JFK or ORD and then connecting to Cathay Pacific via HKG.

Or you can go via the Middle East on either Emirates, Etihad or Qatar.

I don't think there's necessarily any one "best" option. Depends on class of service, what you value, if you're using miles or cash, etc.