The choice of Subject Test deserves careful consideration, as the right subjects will enhance your application whereas the wrong ones won’t help as much. For example, the College Board offers Subject Tests in 9 different languages, none of which are likely to help a student get into a competitive engineering or physical therapy program.
Generally, students should choose subjects consistent with their intended major, assuming one exists. Students on a pre-law track would do well to test in World History, U.S. History, or both. Science-minded students can choose from chemistry, physics, or two different versions of biology.
Students should also prioritize subjects in which they excel; the only Subject Tests that improve chances of admission are the high-scoring ones.
The sole exception to tailoring your subjects to your intended major is Math. Just about every school that requests SAT Subject Tests looks for Mathematics. Your choice in this regard will be which level to take: Level 1 or Level 2. Math Level 1 covers the same topics found in the math sections of the SAT or ACT. Math Level 2 includes those topics and adds advanced trigonometry and precalculus. Students on a science or math track should strongly consider taking Math Level 2.