What are the best transportation options from Don Mueang Airport to Ayutthaya?

You will see that most trains are "3 digit" train numbers.

That means they are "ordinary" trains. Commuter between cities for short distance use.

Trains w 2 digits or 1 digit are usually Express, Rapid etc and dont stop at every station. Also Rapid trains usually have sleeper cars that many people use to head to Chiang Mai or Nong Khai.

12go or the SRT website do NOT sell online for Ordinary trains since the fare is usually 20-40 baht or so... and the processing fee of 30 baht would be higher than the fare itself.

Just buy it at the station. They are never full since its a "seat configuration" train (vs a sleeper car etc)

Take a taxi directly.

Yeah the train stops just outside the airport and goes straight up to ayutthaya about every hour daytime hours. Like 40B and will take about 2hrs. Easiest way to do it by far.

Whatever you do DON’T get into one of those vans. The drivers are suicidal.

Train.... there’s a station near the airport

Taxi - easy

You can check the schedule on the srt website. If the train don't work for you, grab a taxi to Future Park. Vans leave from the parking lot every fifteen minutes or so. You can also get vans at Mochit BTS, but this is a slight bit of backtracking