Withdrawn 5000 bath from an ATM in Bangkok recently as I ran out of cash and Maybank charged me RM536. After deducting the RM10 processing fee, the exchange rate turns out to be 10.52, and with the RM10 fee the rate is 10.72. The MidValley exchanger rate on that day is 10.6. Since the processing fee is fixed, the more you withdraw, the rate will be better. If you intend to exchange more than RM1000, then ATM might be your better choice as you no need to travel far to MidValley to get the good rate. But beware, please only withdraw from international banks' ATM as local Thai banks will charge you extra 150 bath for their processing fee. I used Aeon ATM, which is widely available e.g in Big C, Terminal 21, etc.

I will share more tips in BKK when I have more time later.