There is no definitive answer to this question as it largely depends on personal preferences. However, some lip shapes are generally considered to be more rare or unique than others. For example, lips that are very full and plump are often considered to be quite rare. This is because they are not as common as other lip shapes, such as those that are thinner or more average in size. Additionally, lips that have an asymmetrical shape or that are significantly different in size from one another can also be considered quite rare. Ultimately, it is up to the individual to decide what they consider to be the rarest lip shape; there is no right or wrong answer.