ZAGG Glass Curve Elite
ZAGG's is a pioneer in screen protection. They were one of the first companies to ever offer these types of products, and they still have some of the best. Surely you've heard of ZAGG a few times, right?
The new InvisibleShield Glass Curve Elite is the Galaxy S9+ screen protector you want. They test multiple Galaxy S9 phones (because they're not all the same, even from Samsung) and check the curve of the glass on each one. Then, come up with an "average" so the tempered glass screen protectors they sell will fit better than the competition. That's because even Samsung's curved glass isn't the same on each phone. They have small varying differences.
ZAGG works out those variables and has a great screen protector available, and cheap film if you want one of those. With ZAGG you get smooth tempered glass that resists fingerprints. It's scratch and shatter-proof and has a shock-absorbent layer on it for added durability. They make some of the best glass, but it's also the most expensive.