Ideally, your response to the question should reference some elements of the job you're applying for. For example, if the position is a customer service job, you might say that your dream job would have a high level of interaction with customers.
In order to prepare your answer, brainstorm what appeals to you in the job at hand. Do you enjoy solving problems, or mediating conflicts? Do you thrive under pressure? Do you consider yourself a “people person” who likes to engage with clients or with the larger community?