There are hundreds of varieties of summer squash sold as zucchini, but they break down into two main families. Though they can be used interchangeably, each has different strengths.
The familiar deep green cylindrical zucchini tends to have the best flavor, and the darker the zucchini, the better it is. But the flesh can be soft and breaks down when cooked.
The light gray-green slightly bulbous zucchini, which is common at Latino and Middle Eastern markets, has a milder taste but denser, firmer flesh that holds together during cooking.
You may also sometimes see round zucchini, such as Ronde de Nice and Tondo di Piacenza. These are not technically zucchini but summer pumpkins. Nevertheless, they have firm flesh and mild flavor and are very good for stuffing.