The answer
Follow signs down to Basement B1: the Airport Rail Link — 45 THB (~£1), 25–30 min to the city, immune to Bangkok traffic.
Tokens from the machines at B1 (cash — small notes). Trains every 10–15 min, roughly 05:30 to midnight. Get off at Phaya Thai for BTS connections, Makkasan for MRT.
Big bags, family, or landing after midnight? Level 1, official taxi queue: take a ticket from the machine, insist on the meter — 350–550 THB all-in to Sukhumvit including the (legitimate) 50 THB airport surcharge and tolls.
⚠️ The traps (walk past these)
"Meter broken" / flat-rate taxi offers
A driver quoting a flat 800 THB, or claiming the meter is broken, is overcharging you double. Official queue taxis must use the meter by law.
Do this: walk away, take the next cab. Say "meter please" (or "meter krap/ka") before getting in. The 50 THB surcharge and tolls on top of the meter are legit — a flat fare is not.
Touts inside arrivals
Anyone approaching you in the arrivals hall saying "taxi? taxi?" is unlicensed or overpriced. Real airport taxis never approach you — they wait at the Level 1 queue with numbered bays.
Do this: ignore everyone indoors. Level 1, ticket machine, numbered bay.
Arrivals-hall SIM counters
The AIS/True/dtac counters in arrivals push "tourist SIMs" at 299–599 THB. The same networks sell the same data for half the price in any city 7-Eleven.
Do this: use free airport Wi-Fi to order a Grab/check in, buy your SIM in town — or install an eSIM before you fly.
Arrivals-hall currency exchange
The counters right after customs give some of the worst rates in Bangkok.
Do this: if you need baht, walk down to the basement (B1) — the exchange counters near the ARL entrance (Superrich and friends) are famous for near-city-center rates.