Wind drag and motor for me. My top speed is that which I can achieve before I run out of juice pushing my body through the air. Since I'm not a super strong rider, I probably don't ride at speeds where the other factors come into play. Then hills.

What I know for sure is that I ride slower into a headwind. Therefore air resistance is a major factor. I ride slower going uphill.

In other words, the top factors are the ones you'd guess by just thinking about the physics for 2 minutes.

I ride with an upright posture, by necessity: "My neck don't bend that way no more."