Recommend your favorite unknown/underrated indie games here!

Splasher
Song of the Deep
Sideway
Rollers of the Realm
GNOG
Dominique Pamplemousse
Press Any Button
Bad Rats
Skyrim

Oxenfree is probably the one I was most impressed by, going in blindly.

Here is a hidden gem. Not available more at Steam, but the developer gave us for free to immortalize his game.
https://farsky-interactive.itch.io/the-free-ones

Hypnospace Outlaw is just simply amazing. It's not for everyone, since it's a point-and-click web simulator. Still, it's one of my favorite games that I've played: great music, funny dialogue, and brilliant storytelling set in an alternate 1990s web mean to replicate 90s webpage sites like Geocities.
https://store.steampowered.com/app/844590/Hypnospace_Outlaw/

The Coin Game

It began as a small arcade filled with simulations of ticket redemption machines from a solo developer and has grown and continued to grow while in Early Access. It now has three arcades and extras spread across a small island, and as the updates keep coming I expect it's likely to have even more by the time it reaches a full release.

Onirism - A SEVERELY underrated Third-Person Shooter/3D-Platformer hybrid. Still in early access and still more game levels to come, but it already has more content than many full games.

Submachine: Legacy - A refined and HD-remastered collection of the point-and-click series "Submachine". Contains "Submachine" games 1 through 10, "Ancient Adventure", "32 Chambers", "Future Loop Foundation", and new content tied throughout the ten main games to a new huge set of rooms.

INFRA - A first-person puzzle adventure game where you play as a structural analyst, surveying abandoned infrastructutre (hence the name) for structural damage while slowly uncovering evidence of political corruption at the city level. What ending you get is determined by how thoroughly you do your job as a structural analyst, and how many of the environment-based puzzles you solve (many of which are completely optional and not immediately obvious).

Wuppo - A happy-core (think Kirby, Paper Mario, Yoshi Story) metroidvania where you play as a fuzzball spider thing traveling the world to find a new home.

Forgotton Anne
The World is Your Weapon
The Procession to Calvary
TAMASHII
RB: Axolotl
Boss 101