Apple's Retina display is often lauded as one of the highest quality mobile displays. It was originally manufactured by LG and features pixels so densely packed (326 PPI), that the unaided human eye cannot distinguish individual pixels when looking at the screen.
LG just announced a new display panel that may one day dethrone Apple's pixel dense Retina display. LG's new panel is a full 1080p resolution 5 inch screen which works out to a 440 PPI pixel density with a 16:9 aspect ratio. The technology behind this panel is some form of LCD IPS panel which boasts impressive viewing angles.
The panel should be making its way to handset manufacturers sometime during the second half of this year. Hopefully, this display comes with some sort of massive battery capable of powering that many pixels and a massive GPU to handle full 1080p HD content. With the current rate of advancement in mobile devices, this may not be that far of a stretch.