The highest display resolution of the UE43KU6509 is 4K Ultra HD. That's over four times the display resolution of the most common HD type of Full HD. 4K Ultra HD TVs have a resolution of 3840 x 2160 pixels.
A 4K Ultra HD HD type is in fact the best display resolution across all TVs that have been around for a comparable time to the UE43KU6509.
The worst display resolution across similar types of TV is HD Ready.
4K Ultra HD TVs have a resolution of 3840 x 2160 pixels.