Use someone you were accountable to on the project -- not just a friend or a junior co-worker. Although PMI will technically allow you to list anyone involved in the project who can verify your claimed project experience, your application will be judged in part by whether you have credible project contacts who can verify you meet the PMP qualifications. You should assume that PMI will look up your project contacts on LinkedIn, so providing someone with an impressive job title will likely help your application. Also, using business emails and phone numbers (not personal ones) may help your application avoid further scrutiny.