This is probably one of the most common questions about radar detectors. I have read about this and there are always mixed opinions, but for radar the best position is as high as possible to get maximum view of the road ahead. For laser the best position is low as that has a much narrower spread and it is aimed at your license plate and headlights, so lower will pick it up easier.
I have chosen to get maximum radar and thus its up high. Many of the reason people suggest lower is because middle would be 50/50 for radar and laser. Also they say higher is bad for the driver as they cant see it etc. I just want optimal performance.
Escort:
Radar Detector Frequently Asked Questions - Escort Inc.
Valentine1 states in the middle of the window (presumably because of the radar/laser).