When it comes to choosing the best tablet for watching high-definition movies, you'll want to consider factors such as display quality, screen size, audio quality, and overall performance. If you want to watch high defenetion movies, brightness and black levels, as well as color accuracy and range, are equally important factors to consider.
You should check,
Maximum brightness - may vary 368 cd/m2, 430 cd/m2, etc.
Screen resolution - 2,560x1,600 or higher
Pixels per inch - 361ppi or high
Screen size - 8.4-inch to 10.1-inch or above.
Aspect ratio - 16:9
You can buy the new ipad or Samsung Galaxy tablets. There are some other manufactures like Motorola, HP,Sony,producing good tablets.