It's ergonomically correct for your eyes to be in line with a point on the screen that is about 1.97"-3.94" below the top edge of the monitor which would also typically allow you to keep both forearms parallel with the desk. The height of the monitors will depend on the height of your desk and chair and how tall you are, but they should always be at least eye level.
Individual preferences may differ on what height range suits best for work comfort. In general, when monitors are greatly lower than a user's eye level, this requires extra neck flexion that can result in pain and headaches over time. The resulting stress on eyes and upper body is also compromised posture which contributes to physical health problems that often result from prolonged periods of sitting and sedentary lifestyles.