5K in the second generation. Apple uses new Skylake processors for the current iMac Retina 5K version, but also a new graphics card from AMD as well as new input devices. A sophisticated build quality and powerful hardware create a convincing package, but it's not perfect.
http://www.notebookcheck.net/Apple-iMac-Retina-5K-27-inch-M390-Late-2015-Retina-Review.155325.0.html