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I think the 6/6S had the brightest, coolest screens with the best color of any iPhones.

LOVED the screen on my 6.
[doublepost=1559870277][/doublepost]Yesterday I compared my iPhone 6 Plus screen brightness to the latest iPhone XS Max and iPhone XR. The XR was noticeably dimmer to the iPhone XS Max. The iPhone XS Max was noticeably dimmer to my iPhone 6plus. After Comparing some more I found that turning my iPhone 6 Plus screen brightness to half made it the same screen brightness as the iPhone XS Max.
I left the store wondering why there is half the screen brightness at the Max setting on the latest iPhone. The only thing that I can think of and why apple would do that is to conserve the battery.
For now I’m happy with my iPhone 6plus and because of less screen brightness I won’t be upgrading for a while.
 
I managed to brighten mine by going to [Settings], [General], [Accessibility], [Display Accommodations] turn off or move the “Reduce White Point” setting down, I can now see the screen outside

Thank you so much!!! My mom was so frustrated because her screen was dark no matter how far she turned up the brightness on her iPhone 8. This worked perfectly!
 
[doublepost=1559870277][/doublepost]
Yesterday I compared my iPhone 6 Plus screen brightness to the latest iPhone XS Max and iPhone XR. The XR was noticeably dimmer to the iPhone XS Max. The iPhone XS Max was noticeably dimmer to my iPhone 6plus. After Comparing some more I found that turning my iPhone 6 Plus screen brightness to half made it the same screen brightness as the iPhone XS Max.
I left the store wondering why there is half the screen brightness at the Max setting on the latest iPhone. The only thing that I can think of and why apple would do that is to conserve the battery.
For now I’m happy with my iPhone 6plus and because of less screen brightness I won’t be upgrading for a while.

Regarding Phonearenas measurements the peak brightness of the devices you mentioned is:

6 Plus: 500 cd/m2 (nit)
XR: 625 cd/m2 (nit)
XS MAX: 625 cd/m2 (nit)
 
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