My preference is 30-35% on driver/front passenger as well as rear window, and 5% on others (or whatever is stock on SUV's).
I had 20% on a rear window once and got tired of not being able to see out of it at night so had it switched to 35% and was much better.
My wife has night vision issues, so her vehicles get 50% on the front windows, and all her vehicles come with factory tint on other windows (which she feels is a bit dark, but doable for her, and rear view camera helps a lot with that). There are 70% tints out now with better heat blockage now days, and if we ever get another vehicle for her, I will probably do that.
My son got 5% on all windows (except windshield) and within a month was pulled over and received a ticket for the tint.
He had the tint taken off the front windows and replaced with 30%. He admits it makes no difference in heat, and he can see better at night out of it. But it is "not as cool looking" as his 5%.