The type of squat really dictates the proper depth for it. A low-bar back squat will be shallower than a front squat, but the full range of motion for that squat. In such a squat many lifters will be doing it perfectly properly and their quads might only break parallel. That’s because it’s the depth at which the primary movers in that squat hit their strength shortening cycle.
People who are really good at squatting usually do lots of all of them. So I guess my answer would be all of them.
Back squats should be done with a wider stance, and really emphasize the glutes, hamstrings, and spinal erectors.
Front squats really emphasize the anterior chain, I.e., your quads, abs, etc. They’re much deeper, well past parallel quads.
You can also Olympic style squat, which is a very high bar on the back with a significantly narrower stance than a normal back squat. The normal stance lets you go much deeper, your hamstrings should completely cover your calves. Some people’s ability to do this is limited by limitations in ankle flexibility, but you’d an compensate with a raised heel.
I think it is also valuable to do very narrow stance squats on a raised heel ramp. This isolates the vmo, which can get out of balance with the other muscles in the legs.
That being said, it’s not as complicated as that sounds. Even if you’re squatting just once a week you can just rotate which ones you’re doing every 1-6 weeks.