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).
Ok, I'll try and give a more helpful answer (goodness knows it goes against everything I find natural!)...
One problem is that 99.9% of the people responding really won't have tried each of the methods you mention. Even if they have, they are more likely to favor the more recent method they tried as, just by the fact they are trying again, means they were disatisfied with the initial attempt....so I'm not sure how meaningful anything is going to be. This at least goes for the actual courses run by these companies.
Some people will have the materials from all the companies. I do. I used them all. I found the EK ones the easiest to read and they distilled some stuff down nicely. However, there were some areas which I understood well which I didn't appreciate their approach in. This was because I already understood the material at a more sophisticated level and so disliked the 'dumming down' that they occasionally do to make a concept understood. Don't misunderstand me - I am sure those sections were very good for people that *didn't* understand already, but for me they were simply annoying. On the whole I preferred the EK materials but they have some minor drawbacks. I also had the big red book from Kaplan and thought it was thorough but boring. It just presented everything, shotgun style, with no real simplifications and no real condensing to just the necessary parts. I felt I might as well have just gone back and read my entire undergraduate notes. However, the practice questions they provide are useful. I had similar feelings about the TPR materials although I felt there were more errors in the TPR stuff. Their practice questions were still useful though. I also liked the Berkley Review practice exams (a lot).
In the end they all prepare you for the same exam, and none of them predict *well* what will be on it (they claim they do, but they don't - I mean *I* could predict what will be on it as well as they can based on the previous exams and subject content of the MCAT 'syllabus' but really it's to imprecise a prediction to be very useful). You just have to learn the material and do lots of practice questions and the AAMC full length exams, which are the best guide (I used the online mcats). I get bored easily and sometimes confused by a single author so I bought and used multiple sources. For me (and I emphasize "For me") I found EK to be more efficient in terms of time spent for returns. It all depends on your learning style and current level of knowledge. If you have done advanced classes in phys. chem and bio, you may find EK annoying (I just say this as I have advanced Physics and I found EK Physics annoying in parts for this reason - but loved their other books). Everyone on here will have a different experience. I suggest you just go to a bookstore, pick up the books and leaf through them. If you can imagine staying awake with the Kaplan book, buy it. If you think salty the kracker (EK) is hilarious buy that. I suggest you buy a couple. You can always re-sell them once you're done.
As far as books go I used Kaplan's "Comprehensive Review" and the 2001 "Complete Preparation for the MCAT" (lippincott). The Kaplan book was fine, but I didn't think the second was the greatest. Next time (god I hope not) I'd like to take a look at the flowers book. As far as EK goes they seem to be getting a foot hold, but I've never been able to read any of their material. They kinda have a cult following through. Also don't forget about the internet. There are some pretty helpful general info sites and plenty of content specific ones. Whenever you are searching for online help I'd recommend including the word "animation". It helps alot to be able to visualize something.
...and while I was typing Camden typed something very similar...except we concluded different things (I said buy a variety, he said buy one...although I think we probably agree the main benefit in >1 is the practice materials)
The only question that now remains is, Camden is a lawyer....do you trust lawyers?
Boy Wonder has a point about people tending to like the materials they just recently used. In hindsight, I'm pretty sure that I did not put forth the effort I needed to in 1994 when I was taking the Kaplan course (although it's hard to remember that far back ). As a result, I did not perform as well as I now think that I'm capable (we'll see shortly if I was able to do better this time around). This probably makes me a little less of a fair judge when it comes to Kaplan. I still liked the Examkrackers books quite a bit though.