Maui has year-round good weather, and visitors come year-round. The wettest and most heavily trafficked season are slightly different things.
December through March is whale season — humpbacks arrive in force, the beaches are calmer (south and west coasts), and Maui is at its most popular. Accommodation prices peak around Christmas and February school holidays. The north shore and Hāna Highway can get rain, but the rest of the island often remains sunny.
April through June is perhaps the best time: whale season is winding down, rates drop slightly before summer, and conditions are excellent. July and August are school holiday high season — busy, expensive, but excellent weather. September and October are shoulder season again, often the best combination of price and weather, though October can bring the occasional tropical storm.
The upcountry and the Hāna side receive significantly more rain than the west coast, regardless of season.