The best choice among Thailand islands depends on the month and the kind of trip you want. Choose Phuket or Samui for easy first-trip logistics; Koh Lanta or Koh Yao Noi for a slower couple or family stay; Koh Tao for diving; Koh Phangan for social energy plus quiet northern beaches; Koh Chang for a Bangkok-linked nature trip; Koh Kood or Koh Mak for a quieter escape; and Koh Lipe for clear-water scenery when you can accept a longer transfer. For a two-week holiday, stay on one coast and use no more than two island bases.