What is the best and cheapest country for a one-day visa run from Thailand to obtain a tourist visa?

Just go to your embassy in BKK and talk to them. Easier to do it the right way than break the rules. Thailand trying to stop people getting back to back tourist visa to stay longer.

Where are you coming from? Are you talking about a non visa exemption stamp for 30 days all depending on your nationality. You can do a boarder bounce in a day & pay for a 30 day extension once your in Thailand.

I did it recently at the Malaysian border. Hat Yai. to Pedang Besar.

Penang, Malaysia or KL, Malaysia

Do you have an original visa - as in a stamp or a pdf that you got from a Thai consulate before you came, or do you have a stamp in your passport that says “extension of stay”?

If you are in Bangkok just fly somewhere, got a month to get cheap flights Singapore and Malaysia usually best option for most passport holders as you dont need a visa.

First of all, you might want to tell us from where?? Can't really answer without knowing that.

Second, are you asking about a visa run where you go and purchase a visa from a Thai embassy in another country, or are you asking about a border bounce where you just cross into another country and then immediately return?

No Thai embassy in SE Asia provides a visa the same day you apply, so you'll need to spend at least one night in the other country, sometimes more depending on where you go.