By far and away the simplest entity registration to use in writing a business plan is Sole Proprietorship (SP). This is because a Sole Proprietorship is not a separate entity from the person who directly owns the business; however, a small business sole proprietorship owner is directly responsible for its debts. There is no predetermined duration for a sole proprietorship, thus as long as your business plan is in operation, the SP remains in effect. The owner of an SP can also sell the business as he or she see fit, adding to an SP's flexibility.
There are also no formalities when using a SP as the business structure and the business plan must only comply with applicable licensing requirements. The owner (sole proprietor) has total control over the entity. This control is oftentimes subjugated by the risk that is inherent with an SP (i.e., the owner is at complete risk and liability incurred through the acts of the business). The owner's community property is also at risk with an SP.
When writing a business plan, you must look at the future risk of the company along with every other aspect of being in business. A licensed professional can help you to determine if a sole proprietorship is right for you.