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.