High-quality, thick, shiny aluminum cookie sheets are most desirable. For proper heat circulation, a baking sheet should be at least 2 inches smaller that the inside of the oven. The sheet should have very low sides or no sides at all.
Cook's Illustrated tested eight different cookie sheets and found that thick, aluminum sheets provided the best distribution of heat, resulting in evenly browned cookies. Thin, aluminum pans have a tendency to bake unevenly and the pan may warp. Insulated cookie sheets consist of two thin layers with air circulating in between. Unfortunately, with these baking sheets, cookies take longer to brown, sometimes leading to over-baking. Pans with dark finishes, such as non-stick pans, are generally not recommended since they are prone to over-browning due to absorbing heat more quickly.