Spigen is one of the most popular brands over the past 3-4 years due to the quality and quantity of cases they provide. Almost any shape, size, color, and material is available for the Galaxy S8. Not to mention the sleekest and light thin case like this one, to extremely durable and rugged cases. They have it all.
My favorite though, is the Liquid Crystal lineup from Spigen. It comes in matte black, and also a crystal clear version.
This is a simple, thin, lightweight TPU case that's flexible and easy to install. Just bend the sides and snap your phone in place. It's also extremely cheap, but made well enough to fully protect the phone. The sides even wrap around the edges just enough to keep the screen safe while laying face-down. If you're to get one case and are on a budget, we'd start here.
Buy it on Amazon for $10.99