I have not studied any MCAT material yet but many have suggested to take one diagnostic test to see where I'm at. I know some tests (purchased full lengths) are much more valuable than others so can someone recommend me an exam that I could use before I start studying? Thanks all!
A diagnostic exam is just that, a diagnostic. your score should not matter so much as what the test tells you about your current state of readiness and MCAT familiarity. Most places offer a free diagnostic, but, as was eloquently stated above, no 1 MCAT exam, even if it was 2,3, or even 4x as long, could accurately diagnose your entire MCAT acumen.
Rather, take one of these half length exams (I would not burn a full one as a diag) as a chance to se how YOU handle MCAT style questions and he MCAT way of thinking. I have found in many students there can be a big disparity between the science you are good at in undergrad, and the MCAT science you do well on, MCAT difficulty is all about how the exam tests the science.
Use the diag as a jumping off point, you are going to study all the science anyway, but it will give you a rough idea of your current MCAT thinking ability. Use some content quizzes to assess your "science" acumen. You will be diagnosing as you go through each area of study, its a marathon not a sprint. If you are strapped for study time, prior to reading a chapter or watching a video, take a 15-20 Q quiz on that content to see if you need to study that content thoroughly (if you do poorly) or if you can jump right into MCAT style questions on that science (if you do really well).
The AAMC official guide test is a great source for a diagnostic, and it introduces you to the AAMC style and format.
NextStep also offers a free diagnostic as well as a few other companies. These will introduce you nicely to the exam and offer a wake up call for recognizing the difference between undergrad science and MCAT science tests.
Hope this helps, good luck!