Contrast is emphasis. The sharpest contrast should be where you want the eye to go first. Contrast comes in many forms, including value, color, detail and line.
The most important element is the value; the most influential of the principles is contrast. Put them together and you have a kind of contrast that packs the most punch.
This usually refers to contrast in hue or temperature, but intensity also can play a role. A power struggle ensues when hues are equally intense. One must dominate, or the eye won't know where to look.
Contrast in Complexity
Just as brightness is enhanced by neutrality, detail, texture and pattern are more exciting next to areas of simplicity.
Contrast in Line
A rendering with entirely horizontal lines is effectively no rendering at all. When even a minimum of diagonal and vertical lines is added, the improvement is dramatic.