What is the experience of obtaining an ED visa from the Thai embassy in Seoul, and where is the best embassy for this purpose in Asia?

There is no "best consulate" to use for an ED visa. Your school should be recommending where you go.

EVERY consulate now is gonna look at your previous entry history to thailand so if you've been living here on back-2-back tourist visas or visa exempt entries for a while and now want an ED visa they're going to look at you with a critical eye thinking you don't really care about studying and are just trying to further your stay here.

Have the documentation correct from the MOE and from your school, pick a consulate, look on their website to see what's required, meet those requirements and get the visa. DON'T meet the requirements don't get the visa it's NOT brain science or rocket surgery.

Better to talk to your school. Often school have experience with specific consulates and could give you an advice.

The best Thai consulate in my opinion is still Vientiane. They take Thai baht so no need to exchange currency. Right next to Thailand so you can go back the same day even. You can take the bus from the south bus station to the friendship bridge for less than a dollar.

Theres a few caveats but workable.

1 ED visa every 12 months. Have to make an appointment, which actually makes the consulate run more efficient.

I just moved here from Korea. I got my ED visa from the Thai Embassy in Seoul. No issues. Everything was smooth.