Round, flesh-filled pumpkins are a good choice for cooking. The pulp also has a better texture (less grainy) and is smoother. It's called in American English "Sugar Pumpkin" or "Pie Pumpkin." So, pick some smaller in shape than the monstrous pumpkins found in a typical pumpkin field. Of the American variety Jack O'Lantern, the latter have hollow cavities, lean flesh, and are rather tasteless. They have been designed to facilitate cutting and therefore make Halloween lanterns. This is why they have a thinner shell and generally have less flesh (or pumpkin casings) inside. Plus, they're more grainy and stringy, and they tend to hold more water than smaller pumpkins.