As a general principle in defective product cases, you want to include any and all parties involved in the chain of distribution (the path that the product takes from manufacture to distribution to customers) of the injury-causing product.
The chain of distribution is typically made up of the manufacturer, the retailer, and any number of suppliers, wholesalers, and distributors in between. Depending on your case, the defendants may also include consultants, contractors, quality-control engineers, designers, and anyone else with some sort of connection to the defect in the injury causing product. To learn more, read Nolo's article Defective Product Liability Claims: Who to Sue?