Which word best describes something that is peculiar or uniquely individual?

The term "idiosyncratic" is the most fitting choice because it specifically refers to a distinctive or peculiar characteristic that is unique to an individual or thing. This word implies a level of individuality that sets something apart from the norm. For example, an idiosyncratic habit or interest reflects personal traits or behaviors that are not universally exhibited but are distinctive to that person.

The other options do not convey the same sense of individuality. "Eccentric" might describe someone who behaves in an unconventional manner, but it does not capture the nuance of uniqueness as strongly as "idiosyncratic." On the other hand, "commonplace" and "typical" suggest standard or usual characteristics, which are antithetical to the idea of peculiarity and uniqueness. Thus, "idiosyncratic" is the most appropriate choice for describing something that is peculiar or uniquely individual.