Format conversion is rarely lossless, and what gets lost depends on which direction you are going. Each converter page names its own losses; the pattern is worth knowing before you start.
KML is what Google Earth and Google My Maps speak. Geometry crosses cleanly in both directions. Styling, folders, network links and time spans do not, because GeoJSON has nowhere to put them.
WKT is the text form spatial databases use, so it is the shortest path between a file and a PostGIS table. It carries geometry only: properties are not part of the format and do not survive the trip.
GPX comes off handheld GPS units and fitness platforms. Tracks and routes become lines, waypoints become points, and sensor extensions such as heart rate fall outside the schema and are dropped.
Latitude and longitude covers the most common case of all, which is a spreadsheet with two coordinate columns. Watch the order: people write latitude first, GeoJSON stores longitude first, and getting that backwards is the single most common spatial bug there is. The debugging guide covers how to spot it.
All converters
Into GeoJSON
KML to GeoJSON Converter
Convert KML files from Google Earth to GeoJSON format for web mapping and GIS applications.
WKT to GeoJSON Converter
Convert Well-Known Text geometries to GeoJSON format for web mapping and visualization.
GPX to GeoJSON Converter
Convert GPX track files to GeoJSON format. Transform GPS data for web mapping.
Lat/Long to GeoJSON Converter
Convert latitude and longitude coordinates to GeoJSON Point features.
Out of GeoJSON
GeoJSON to KML Converter
Convert GeoJSON files to KML format for use in Google Earth and other mapping applications.
GeoJSON to WKT Converter
Convert GeoJSON geometries to Well-Known Text format for use in databases like PostGIS.
GeoJSON to Lat/Long Extractor
Extract latitude and longitude coordinates from GeoJSON features into a table or CSV.