Google Earth's format, and the reason it looks good - unlike GPX or GeoJSON it carries styling, so lines have colours and placemarks have icons. Still XML underneath, and still the thing to convert to when someone wants to see a route draped over terrain.

Every conversion below runs in this page, on your own machine. Nothing is uploaded, there is no queue, and there is no account to make. Pick a target format, or just drop a KML file above and choose afterwards.