Note: this post is based on my answer on Quora.
Short answer: (a) people inside the team/organization using the personas, and (b) customers/users the personas are based on.
You want the people who will be using the personas to participate in their creation as much as possible for them to internalize important customer segment information already during the creation process, and to avoid them rejecting the personas because of them being “artificial”.
When being part of the process of persona creation, the persona users are more likely to accept and adopt personas in their decision-making.
Secondly, I mentioned customers because even when creating data-driven personas (i.e., those based on quantitative numbers such as CRM or Web analytics records), you still want in-depth insights on the customers’ pain points, needs, and wants. Those are much harder to elicit using automatic means, so in-depth interviews with 6–12 customers (or more, if you want more than 3 personas) are highly useful.
See relevant persona research here: Persona Research