Which Mileage program is best for this:
 

Seattle - Beijing
Beijing - somewhere in SE Asia (Depensar, Phuket, Cebu?)
Somewhere - either back to Beijing, then Seattle..
or straight back to Seattle.

United miles? AA miles? Delta? some other airline's? what's best?

Thanks!

Is this question about earning miles or using miles?

Look and ask here http://www.flyertalk.com/forum/infor...help-here.html

Sorry, using. i have enough SPG miles to transfer to anything i need. so i'm wondering what's the best system to use to find a ticket for this trip..

Do you want business or economy seats?
You need to do some research on which airlines fly these routes. Then find out if these airlines are partners of any ffp's if they are not a major airline that you can transfer miles to ( AA, UA Delta etc).
Some ffp's allow one way awards and other only return's.
You are likely to end up with a mixture of ffp's and cash paid tickets.

Mileage Plus (UA) allows one stopover on a round trip so might be good for this routing (SEA-BJS-xxx-SEA), but the exchange ratio is bad (2 SPG for 1 MP mile). I think that AA is point to point only, no stopover, so you would need three one-way awards, on the other hand the exchange ratio is 1:1. SPG has 30 transfer partners for airline points, and learning which is the best is going to take you some time. Each FFP has its own benefits, drawbacks and quirks.

First think about what is important for you - first, business or economy, are you trying to minimize points usage, want non-stop flights or connections are OK, what dates will you travel and how flexible are you with those dates, how many people are going, etc.

Then the simplest way to solve your problem would be to use an award service. I'd expect that they would already have a decent idea about the various FFPs and what the best way is to accomplish your goal. and I am sure they could help out on this if you want to pay a fee. See thread.
http://www.flyertalk.com/forum/trave...e-reviews.html

SEA-PEK (stop) - BKK or HKT (destination) - SIN (stop) - SEA

on AC, its 150K miles in business class, just avoid those with YQ ie Thai / ANA / AC.

For other SEA destinations, just use LCCs out of BKK or SIN.