Which correct word for “posh” and “preppy” in modern Greek: κομψός, κυριλέ or σικ?

Panos Skoulidas‘ answer is right. To elaborate:

• Κομψός means “elegant, clean cut”. It has ancient lineage. It does not explicitly mean that someone is fashionable; it can correspond to “classic”, and it can certainly be used approvingly by an 80 year old.
• Σικ, from French chic, explicitly refers to being up to date with fashion.
• Both posh and preppy are negative evaluations, posh more so. The closest of the three is κυριλέ, which is derisive slang about someone with upper class affectation in how they present themselves (so posh, but without the British connotations, and more about parvenues). It is derived from κύριος, (in this context) “gentleman”, plus the French fashion style suffix -é.
• The /l/ is a random consonant, inserted so the word wouldn’t end up ambiguous with κύριε “sir!”