Originally Posted by D1andonlyDman

ATM's only offer the best rates if you don't have to pay ATM fees on international transactions.

In general, the rates at the airport will get you 40-70 baht less per $100 than the better banks and change kiosks in downtown Bangkok. But smaller bills are better deals at the airport, because the downtown changers give better rates on $50s and $100s than for $5s, $10s, and $20s

Once you are within Bangkok, I find that the various banks and change kiosks are generally all within 10 baht (which translates to around 30-35 cents) of each other per $100 US.