Bistro 25 in Sayville is a real winner (try the lamb porterhouse). For sushi, there's Onsen in Oakdale. And I really like Thai Angel in Islandia.
If you don't have a car and you're staying at the Hilton Garden Inn, Mama Mia's is walking distance and not bad.
Thai Angel Restaurant
1812 Veterans Hwy, Islandia, NY 11749