ProPhoto RGB
A very wide colour space that encompasses almost all colours a camera can capture, including some outside human vision.
In depth
Its size is its purpose: as an editing space it guarantees that no captured colour is clipped before the final conversion. That size also demands 16-bit files, because spreading only 256 levels across such a huge range produces visible banding. It should never be used for delivery, only as a working space.
Examples in practice
Heavy saturation adjustments on a sunset stay smooth in 16-bit ProPhoto and band badly in 8-bit.