In terms of similarity to the actual exam, I would recommend the following in this order:
1. AAMC
2. Kaplan
3. NextStep
4. Anything else.
It is best to center your practice on the practice problems from the AAMC. You want to develop an AAMC mindset for test day and not a Kaplan, NextStep, or Princeton Review Mindset. For this reason, I would recommend using all the AAMC Practice Problems and then supplementing with a variety of exams from a variety of companies.