If your trip continues to Phuket, Chiang Mai or Samui, tell us before we quote. A BKK long-haul plus a DMK domestic on a separate ticket is a classic way to lose a day. Through-tickets on Thai, a Gulf carrier, or Singapore Airlines with a Bangkok stop behave differently from two internet specials.
Suvarnabhumi
This is the default for Europe, the Gulf, Australia, Japan and most US routings. The airport rail link is real; so are taxi queues after several widebodies land together. Late inbound plus a same-night hotel in Sukhumvit is fine. Late inbound plus a 06:00 Samui flight is not, unless it is one ticket with a legal connection.
Don Mueang
We will use DMK when that is the honest airport for the carrier. We will not quietly write “Bangkok” and let you discover the other side of the city at 01:00.
Seasons
Cooler, drier months around November–February pull leisure fares up. Songkran week is its own spike. Monsoon does not close BKK, but it does delay the last regional hop — another reason not to self-connect.