An open star tip is ideal, since it will give your churros their signature ridges, but don't just use any star tip. I tested several and found Wilton 1M to be the best for piping churros. The large piping tip has fewer deep spikes, which creates deeply ridged churros that cook evenly. Star tips with many short spikes, like Atecco 8B, create churros that are too thick and likely to explode when baking, and difficult to cook through without over cooking the exterior. If you don't have a 1M piping tip, I recommend purchasing one. They're affordable and great for lots of uses, like piping frosting or meringue in a simple rose shape. Check out my Strawberry Meringue Roses to see it in action!