Thailand answers from people who actually live there

Travellers consistently say there is no single 'cheap' Thai ATM for foreign cards in the usual sense — almost all banks charge the same flat withdrawal fee, which has crept up over time from 220 to 250 baht, and some report certain machines (often Mastercard-linked or specific banks like Krung Thai, Bangkok Bank, or Siam Commercial) now charging as much as 350 baht, so it pays to cancel and try a neighboring machine if one quotes a high fee. The one outlier people repeatedly name as cheaper is AEON, which has charged 150 baht, though AEON machines are rare and hard to find. Since the fee is flat, people recommend withdrawing the maximum allowed per transaction rather than small amounts — Krungsri (the yellow ATM) is widely cited as allowing up to 30,000 baht, more than many other banks' 20,000–25,000 baht limits. Everyone stresses declining the ATM's own currency conversion offer, since accepting it gives a much worse rate than letting your card network convert. Some travellers use cards like Charles Schwab or Fidelity that reimburse these ATM fees afterward, which doesn't lower the Thai fee itself but offsets it.