UWorld is a must. Use this as a learning tool, not just an assessment.
Secrets as a quick review/wrap up a week before the exam.
First Aid or Step Up for your text source.
However, if you've been following any of the threads regarding people having just taken the tests in the last 1.5 months, no one is sure what's the best source. We've all felt the CK exam, being "changed up" and all, was not what we were told to expect (by those who have taken it in previous years).