Are there any systems that are more high-yield than others on step 1? I'd like to know so that I can devote more time to those systems that will provide greater return on my score.
No, there are not. It's all balanced. Seriously. People have warped memories that tend to focus on what they were least prepared for.
You can Google the breakdown, but the biggest areas are pathology, physiology, pathophysiology, microbiology, and biochemistry. Sounds pretty bland and non-specific, right? It is.
Anatomy and its cousins (embryology and histology) are not a very prominent part of the test.