Depends a ton on the position.
Backshots(Doggy Style for those unfamiliar with the slang) - My eyes are focused primarily on her jiggling ass with secondary focus on the back of her head or face if she's looking over her shoulder. When I'm pulling hair, I lay a little more focus on the back of her head.
Missionary(or anything front facing) - My focus is all over, but I believe primarily it's on her face. Secondary would be on the penetration I suppose.
Her laying on her side and I am standing(Sorry, I don't know what this position is called) - Her face, breasts, hips and ass get a ton of focus. To ensure I climax, I focus primarily on the latter.
To delay climax, I try to focus away from her body.