Detecting round shapes among contours

Question

I have a code, which gives me a lot of contours suspected to be a ball, In order to minimize the list and find the real ball among them, I have used a couple of classifiers. First Circularity, I use a formula which shows how round are the contours, The roundest are chosen for the next step. Next we compare an Image of a real ball to the ball we are searching for in the image. This is enough almost 85% of the time , yet for our needs we need something even better. Are there any other good classifiers? we are using opencv on c++