what is a good serve that is not returnable

A good serve that is difficult to return is a well-executed "spin serve." This type of serve involves imparting spin on the ball as you hit it, causing it to curve or change direction in mid-air. This can make it challenging for your opponent to predict the trajectory and effectively return the serve. Additionally, serving with a combination of power and accuracy, such as hitting the serve close to the lines, can also make it more difficult to return successfully.