GIMP's native format which will preserve all image data, including layer, the current selection, channels, transparency, paths and guides. Note that it doesn't save undo history.
XCF images take up a lot of disk space, and are not supported by most image viewers, which is why XCF is more suitable for saving images for future editing (rather than image exchange).
If you must exchange a XCF file, you can use the compress option, which uses either gzip (using xcf.gz/gz/xcfgz extension) or bzip2 (using xcf.bz2/bz2/xcfbz2 extension) compression.
Other GIMP's native format includes GIMP Brush (.gbr), GIMP Animated Brush (.gih), and GIMP Pattern (.pat).
