The first decision isn't which island – it's which coast. Thailand's islands divide into the Andaman Sea (west) and the Gulf of Thailand (east), and they run opposite monsoons. Nail the coast for your dates, then pick the island for your vibe.
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Add it in the trip planner →The Andaman coast (Phuket, Krabi, Phi Phi, Koh Lanta) is best November–April and reached via Phuket or Krabi airports. The Gulf coast (Koh Samui, Koh Phangan, Koh Tao) is best roughly January–April and August–September, reached via Samui airport or ferries from Surat Thani/Chumphon. If you're travelling in October, favour the Gulf; in February, either works. See the full picture in our best-time guide.
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Less than you think. Ferries eat half-days and you spend the trip packing rather than relaxing. For a two-week trip, pick one island zone and settle, using day trips for variety. If you must combine, keep it within one coast – Phi Phi and Lanta pair naturally, as do Phangan and Tao – to avoid long, weather-dependent crossings. See getting around for ferry logistics.
There's no single winner – it depends on your dates and vibe. For comfort and ease, Koh Samui; for scenery and adventure, Krabi/Railay; for diving on a budget, Koh Tao; for a quiet beach week, Koh Lanta. Match the island to your trip rather than chasing a "best".
Let your dates decide. The Andaman (Phuket, Krabi, Phi Phi) shines November–April; the Gulf (Samui, Phangan, Tao) is better January–April and August–September, and is your safer bet in October when the Andaman is wettest.
For a typical two-week holiday, one – maybe two if they're on the same coast. Ferries and transfers burn time and money, and settling into a single island beats a rushed hop every two days.