Prep books usually avoid listing high-yield topics because it's risky - after all, even the most commonly tested topics might not appear even once on a given MCAT, while lower-yield topics like momentum and the skeletal system could easily appear in a passage. That being said, here's a rough list of topics that come up all the time (and I'm sure others will add to this):
Physics - fluids, optics, sound, circuits. Basically the topics that are easiest to relate to the body. Other concepts like translational motion were more common on the old MCAT, but certainly aren't out of the question now.
GChem - acid/base (REALLY important), solutions, periodic trends (surprisingly higher-yield than one might think), electrochem. But general chemistry topics all relate to each other, so it's hard to pick any out as being lower-yield - these are just especially important. Le Chatelier's principle also comes up frequently.
Bio - this is a bit harder due to the "new MCAT" focus on the interpretation of experiment-based passages. DNA / RNA, prokaryotic vs eukaryotic cells, membrane transport (active / passive / facilitated / osmosis), and Mendelian genetics have been appearing all the time. Hormones, the kidney / excretory system, and most other body systems in general have always been important. But those are just content topics that are good to know inside and out - certain passage topics come up often too, like G-protein coupled receptor systems, certain diseases (like cystic fibrosis), and certain DNA/RNA experiments (especially miRNAs, RNA interference, etc). You don't need to know much background about these topics, but if you incorporate scientific journal articles into your study plan, you could definitely benefit from reading ones that focus on these topics.
Ochem - this one tends to be overlooked, since the new test has reduced its overall focus on organic chemistry. But certain things are almost guaranteed to appear: functional groups, saponification, types of isomers, and laboratory techniques (both "simple" ones, like distillation and recrystallization, and "complex" ones, like NMR). Sn1 vs Sn2 was historically very high-yield on the old exam - I haven't seen much of it on the new test, but it couldn't hurt to be familiar with the principles.
Biochem - AMINO ACIDS (probably the highest-yield topic of ANY section). Enzymes and inhibition / Km / Vmax, glycolysis, and analytic techniques are also important.
Psych - this one is a bit more unpredictable. Though some concepts are tested nearly all the time (James-Lange vs Cannon-Bard, fundamental attribution error, etc), to be safe, you want to be familiar with every term you see in your prep books or on practice tests.
20 (
