This Apple monitor is about as good as you can get currently, although you're paying a premium for the Apple name.
PPI (pixels per inch) is simply the number of horizontal pixels (1920) divided by the display width (20.35"). It's a measure of pixel density, nothing more. A lousy 15" monitor could have the same density value.
The Apple 24" resolution is 1920x1200. If you really want the best, get a 30" display instead, which has 2560x1600 resolution, and the same pixel density value.
Keep in mind these Apple displays cannot be easily connected to Windows machines, only Macs.
