Phuket has two seasons. November through April is dry, sunny, and busy — Christmas and New Year are the absolute peak, with prices doubling and beaches packed. February and March remain dry with slightly thinner crowds and the best weather of the year.
May through October is the southwest monsoon. Rain comes in fierce, short bursts rather than all-day drizzle, and most days still have plenty of sun, but the sea is rough and the red-flag rip currents at Patong Beach kill swimmers every year — take the warnings seriously. Hotel prices drop by 30-50%, and the town feels different: easier to walk, easier to find a table, much greener.
If you only want to come once, target late November or March. If you don't mind some rain in exchange for a calmer experience and lower prices, the shoulder months (May-June, September-October) work well.