To be honest, you can't go wrong with any of the three you mentioned as long as you put in the work. You will hear people advocate for all three of these as being the best. I have the benefit of having taking a Kaplan course the first time I took the MCAT (way back in 1994), and then studying on my own with the ExamKrackers books for the this year's August exam. To tell you what worked better, we'll have to wait until later today (or possibly tomorrow).
But I will say this: a lot will depend on how you want to study. Do you want to take a course or study on your own? If you want to study on your own then I would buy the examkracker books. I actually preferred them to the Kaplan books (but to be fair, the only books used in the Kaplan course that I've seen are eight years old). If you study on your, in addition to the examkrackers books, buy AAMC practice tests and materials, and the examkrackers exams. You'll want to take practice exams under real test conditions. That's a big part of your improvement. This should be plenty of exams (it's all I did), but if you need more you can buy Berkeley Review and Columbia Review practice tests online. I think their tests are as good as EK, TPR, and Kaplan (but nothing compares with AAMC). This will be more than enough tests.
If you decide to take a course, I think a big determining factor should be the instructor. That can be the biggest benefit to a course, but often times the instructors are not stellar. If you find a good one I would go with that.
Finally, when you're reviewing the material, just pick one company. All of the ones mentioned above will provide you with all the review you need, and it's a waste of money to buy more than one prep company's review materials. However, it's fine to take different companies practice tests (but I'd stick to the one's mentioned above and avoid the typical bookstore crap, like ARCO or Barron's).