It's hard to narrow it down to one resource, given the relative importance and complexity of the MCAT (it's multi-subject). However, I will say that if I were to take it again, I'd probably buy all of the old AAMC tests and take them all. Assuming you have a good baseline of knowledge, a major barrier to overcome, for many, is in being familiar with the test, diagnosing and solving remaining holes in knowledge, developing good test-taking strategies, timing, and in ironing out any test-taking issues.