I use the Huawei Mediapad M3 (not Lite) 8.4 inch. It is known to have a very bright screen compared to other, cheaper pads. It is usable in bright sunshine, but certainly not as bright as the Phantom 4 Advanced + with built-in screen, which I also use.
The M3 8.4 also has very high resolution screen. I don't know if it is the brightest, though. The high-end Samsung S-pads have AMOLED screen with high contrast, that helps in sunshine.
Open shade is your best friend. It's cooler, keeping you and your tablet happier, and the shade makes any screen readable. Even an umbrella works great. If you are flying by FPV, even a black cloth over your head will work nicely, like the old view camera photographers used, until you need to land, when you toss it aside.