aparta

Aparta runs along two lines at once: set apart from the rest, and therefore singled out. Context sorts them, but the adverb aparte is more clearly the first sense, meaning separately or on its own, while speciala covers special where no separation is implied. The verb aparteni, to belong, sits in the same root family and reverses the logic rather neatly. Apartigi is to separate things, apartiĝi is to come apart, and aparteco is distinctiveness. When you want the strong emphatic sense of exceptional, eksterordinara is the safer word.