There are so many Sonic Slush flavors to choose from and the best is a matter of opinion. Some of the most popular flavors include Blue Fruit Punch Slush, Watermelon Slush, Strawberry Slush, Cherry Limeade Slush, Lemonade Slush, Raspberry Blue Slush, Sour Apple Slush, Sour Cherry Slush, and Lemon Lime Slush.