There is no "best place to do it" there's ONE place to file a 90 day report in person in Bangkok on a DTV and that's at the main immigration office out at the government complex Building B, Section A inside the immigration office
you need the TM47 form and using the address where you have a TM30 filed (they will sometimes check that you have one on file)
OR
You go to Bumrungrad Hospital on the 10th floor in the Business Center, and they'll do a 90 day report for you for 650 baht. You leave your passport with them, they send the runner with a PILE of passports, do the 90 day report then you can pick your passport back up
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