IQShield LiquidSkin 2-Pack ($8)
A lot of screen protectors for the Galaxy S8 didn't work very well and were eventually removed from Amazon or online shops. One that didn't have that problem was IQShield.
This company uses a unique wet-application with its "smart film" to ensure the best fit for your Galaxy S9 display. It's not tempered glass, but that means you'll get a perfect installation and plenty of protection without breaking the bank.
You'll get a proprietary LiquidSkin screen protector film, installation tray, lint cloth, and a squeegee to get rid of bubbles. Then, if anything goes wrong the kit comes with two so you have a backup. This is one of the highest rated screen protectors for the Galaxy S8, and so far we're seeing solid reviews for the Galaxy S9 variant.
Or, get a Skinomi film, which has great reviews.