geniulo

An honest decode, including the awkward part. Geniulo is genio plus -ul-, the suffix for a person characterised by the root, so literally a genius-person. The awkward part is that genio already covers the person in ordinary use, exactly as English genius does: Mozart estis genio is unremarkable Esperanto. So -ul- here is not filling a gap in the language, it is removing an ambiguity - and that is a real thing suffixes do, even when the base word could have coped.

When the ambiguity is worth removing. Genio runs three ways: the faculty (li havas genion), the person who has it, and, in older and translated literature, a spirit or genie in the Arabian sense. Geniulo can only be the person. So writers reach for it when the sentence would otherwise wobble - la geniuloj de la epoko, the great minds of the age - and leave genio alone when the context is doing the work. Neither is wrong. Reaching for geniulo every time reads slightly heavy; never reaching for it costs you a distinction.

The neighbours, in ascending order. Kapabla is competent, talento is a specific gift, inteligenteco is the general quality of a quick mind, and geniulo is the top of the ladder and should be spent sparingly. The adjective is genia, and geniaĵo is a stroke of genius - the thing rather than the person. The -ul- suffix builds dozens of these; this is one of the few where the base word could have done the job alone.