Generally the best place to mount a detector is up high by your rear view mirror. That gives it the best view of the road and thus the best performance, you can hide it behind your tint strip so it’s less visible from the outside, and it leads to a cleaner install when you hardwire the detector.
Another good option is to hang it under your rear view mirror using a Blendmount. This is helpful for areas where it’s illegal to attach radar detectors to your windshield, plus it avoids suction cup failures leading to your radar detector falling and hitting your dash.
You can also mount it down low on the windshield, but you may see slightly less range in certain situations and it’s often harder to reach your radar detector.
I don’t recommend mounting near the A-pillar because the windshield’s curvature leads to the detector angling off sideways.
Pro-tip: Be sure to angle your radar detector straight ahead for best performance. Don’t point it to the sky like you’re trying to pick up the International Space Station. 😉