Ok well here I go, I see a ton of views in this section but no comments.
First of all, I'm coming from a Moto Z2 Play and a OnePlus 3T:
The screen brightness of the OnePlus 6T is the major letdown I ever had, it is miserable, sad, despicable... Here is the deal, if I set the screen to 100% brightness in the O6T, it equals to anywhere around 85% or less of the max brightness in my 1 year old Moto Z2 Play !! also, the OnePlus 3T is brighter than the O6T by a tiny notch, but I have to add that my OnePlus 3T is 2 years old and noticeably lost its max brightness, but it still beats the O6T!!!!
If you are wondering about my device, I went to 2 T-mobile stores in the area and compared brightness, they are the same as my unit, so it is safe to say that the O6T is a disappointing device when it comes to screen brightness...
I will edit this post if I get more info or if I need to correct something.