Warning: I have zero experience in sales, though I sometimes read sales/marketing books for fun. I personally prefer sales books to marketing books because marketing books tend to overuse jargon.
You yourself must believe that the product that you are supposed to sell is the #1 product in the world. If you can't convince yourself, count yourself lucky if you manage to convince customers to buy this product. Unless you are 100% sure that badmouthing rival products will work, emphasize how good your product is rather than emphasize how bad rival products are. Remember that you are degrading yourself when you degrade others.