TL;DR:Use BTS/MRT and river boats for the fastest city travel; taxis/Grab and motorcycle taxis for flexible last-mile trips; trains, long-distance buses or domestic flights for travel beyond Bangkok.
The fastest, most reliable way to get around central Bangkok is the BTS Skytrain and MRT subway — they avoid gridlock, link the main business, shopping and transit hubs, and use stored-value or contactless payment (BTS: https://www.bts.co.th; MRTA: https://www.mrta.co.th). Use the Chao Phraya Express and Khlong boats for riverside destinations. Taxis (use the meter) and Grab provide door-to-door convenience; motorcycle taxis are the quickest option for very short trips. Buses cover the whole city but are slower and less predictable. For travel beyond Bangkok choose trains (State Railway of Thailand) for regional and overnight routes, long-distance buses from major terminals (Mo Chit, Ekkamai, Sai Tai Mai) for budget travel, or domestic flights from Don Mueang and Suvarnabhumi for speed (SRT: http://www.railway.co.th). Practical tips: keep a stored-value card or contactless bank card, install Grab, carry small change for taxis/buses, and avoid driving yourself during peak hours.
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