I had a Galaxy Tab S 10.5 (AMOLED screen) and a 2017 Amazon Fire 10" (LCD screen). Video image quality was
substantially better on the AMOLED screen. The dog destroyed the Tab S (knocked it out of my hand and screen shattered when it hit the ground). I've been using the Amazon Fire, but despite having what reviewers say is one of the best screens on an Android tablet, it's vastly inferior to the AMOLED screen on my broken tablet. I plan to get a Tab S5e as soon as they become available ($399 MSRP). (Avoid the Tab S2 - its second revision has not been approved by Hollywood so it will not display Netflix, Amazon Video, etc in HD.)
There have been some very good sales on the 2018 9.7" iPad for around $250. That would be a good choice if you also wish to do non-movie things with it, or have already purchased a bunch of iOS apps. But the 4:3 aspect ratio will hurt it when playing videos (will be equivalent to a 8.9" 16:9 screen).
But if you want something cheap
only for viewing videos, consider the 8" 2018 Amazon Fire. It's not the best tablet, doesn't have the greatest screen, but it by far gives you the best price to performance ratio. It's $80 at full price, and you could get it for as low as $40 on sale during the Christmas holiday (the 2017 version went as low as $20, but is now sold out).
https://www.notebookcheck.net/Amazon-Fire-HD-8-2018-Tablet-Review.365771.0.html
Whatever you get, make sure it's a recent tablet. Unlike laptops or desktops, I do
not recommend buying used unless it's only 1-2 years old. The reason is that the tablet processors do not have enough horsepower to decode video streams in real time. They have to rely on the hardware decode built into the GPU. But hardware video decode for the newer video codecs like h.265 and VP9 are only available in newer tablet GPUs. So for a satisfactory streaming experience today and in the near future, you will need a newer tablet.