The best piping tip to use when creating a pumpkin design largely depends on the level of detail desired for the decoration as well as personal preference. A popular choice is the Wilton #12 tip which provides medium-sized round dots suitable for most pumpkin designs due to its versatility in size and ease of use when piping smooth curves consistent with rounded shapes like pumpkins. Another option would be to use Wilton tip #21 or #22 which have slightly larger openings that could produce bigger dot sizes, ideal for larger cakes or cookies. For more intricate designs, a fine tip like Wilton #2 or #3 could be utilized to pipe intricate details such as lines and grooves on the pumpkin's surface. Experimenting with different tips can help achieve various effects and styles in creating your icing pumpkins.