I love Indie games, my personal fav is Dead Cells, because the controls are precise and the gameplay loop is awesome.

In my opinion Steamworld Heist and Furi are underrated.

Would gladly hear about your opinion and picks, thanks.

Too many to choice from... so some mentions go to:

Creeper World series - https://knucklecracker.com/

TRI: Of Friendship and Madness - https://www.tri-game.com/

MOTHERGUNSHIP - http://www.mothergunship.com/

Infested Planet - http://www.rocketbeargames.com/infestedplanet/index.html

Gemcraft: Chasing shadows - https://store.steampowered.com/app/296490/GemCraft__Chasing_Shadows/

and more....

Underrated is a pretty big and at the same time somewhat meaningless word. I mean all the games you mention are rated highly here and everywhere else. They aren't talked about as much as AAA games, but then again, that's because they're indie games. And so on. So I can't really say what's underrated or not, but the most recent of my personal favourite indie games were:

A Short Hike
Short sweet exploration game about climbing a mountain. No combat, just a little relaxed 3D platforming, searching for treasure and doing little quests. Very kid friendly, too. And good music.

Supraland
First person puzzler (think Portal, Q.U.B.E. etc.) and semi-open world exploration game, where the world is quite literally a sandbox. Very few handholding, satisfying progression curve, a couple of nice gadgets and tons of upgrades and secrets to discover. Has a little bit of Zelda or twin stick combat in it too, although that's the weakest aspect of it, IMO. Still, I haven't had this much fun with a game for quite a while. And it has a free demo which is what got me immediately hooked on it.

Filtering my owned GOG games list for Indies, these strike me as absolutely fantastic games:

Aragami
Shadow Tactics
Tetrobot & Co.
The Marvellous Miss Take
Trine 1 & 2


I never thought of Shadow Tactics as an indie, because of it being so ambitious, slick and polished.

Tetrobot and Miss Take seem somewhat unknown and underrated.

Well, Ascendant doesn't get a lot of attention, but I love it and have played it more than I care to admit. Great game.

Project Zomboid

Never heard about them, can you elaborate what you like about these games and what makes them special?

@Kelefane
I heard, that it is the best Zombie apocalypse game around, is that true?

Tetrobot & Co. is the sequel to Blocks That Matter, btw, which is also a very fine 'old' indie game and the reason for my avatar. ;)

Both are 2D puzzle platformers in which you collect blocks in order to build bridges and other puzzle solutions, but there are specific rules for building, like always four blocks at the same time or so. You switch between real time platforming and time-stop building mode.

Ah, I knew I have heard about the name Tetrobot. I played Blocks that Matter on XBLIG on my 360 years ago. Was a good game! :)

Its very deep and detailed. Its a open world sandbox. Massive map. Tons of stuff to do. It makes you feel like you're in a Zombie Apocalypse. But as far as being the best? Its top 5 for sure.

Guns of Icarus, an awesome multiplayer game that died several years ago.

Desktop Dungeons Enhanced Edition - An RPG-lite game with strong puzzle elements and quite addictive gameplay.
Albedo: Eyes From Outer Space - A first-person perspective game with a 1950's sci-fi aesthetic and essentially comprised of a series of 'escape-the-room' puzzles.
Remothered: Tormented Fathers - A giallo-genre third-person game where your character must go about a mansion solving puzzles while discreetly avoiding the owner. The music is perfectly eerie.
The Dream Machine - A really good point-and-click game with graphics comprised completely of claymation and homemade sets.
Signal Ops - A squad-based game with an oil-painting aesthetic where you play a mission operator controlling a squad to accomplish assorted mission objectives via sets of monitors.
Space Pirates and Zombies
Space Pirates and Zombies 2 Both games are fairly distinct in terms of gameplay from one another. The first one is top-down 2D, and the second one is a 3D third-person following-from-behind perspective. They are space action games where you control one or more ships, gather resources, take control of different zones on a world map, and build up and improve your fleet.
Evoland 2 - A jRPG with a time-travel mechanic tied to 'evolving' the jRPG gameplay. Features a good variety of gameplay, and the story is pretty good, with a good length.
Regions of Ruin - A platformer-RPG, with simple graphics and gameplay, but a certain charm to it. In addition to roaming around the country, clearing areas of monsters, you gather resources to improve the town, getting you access to better equipment and abilities.
Infectonator 3: Apocalypse - A fairly casual game where you unleash a zombie virus on different cities, and try to manipulate events to infect every one in each city. As you progress, you are able to improve your zombies so they last longer, move faster, and make them more resilient to attacks.
Honestly, I could go on for quite a while, so I'll leave it at this.

Not my favorite, but the most underrated indie game that I know about is Lost in the Nightmare. Probably the most scariest game I've ever had the pleasure to have my hands on. Its bloody fantastic, more so than many commercial games. Its amongst my "most liked AND most underrated" indie games that I know about but I don't think its THAT underrated compared to perhaps some others.

Tales of Maj'Eyal is one of the obvious games I would take with me to a deserted island. Open-source, copyleft, regular updates and improvements for the past 8 years and probably more years to come, really cheap (or free from the website) and endless replayability with tons of class and build variety.

Friendly and welcoming community as well that you can communicate with and ask for help via the in-game chat, even though the game is singleplayer.

Criminally underrated game.

An indie game that is underrated Zafehouse Diaries or Zafehouse Diaries 2.

It is basically a social simulation of a group of people trying to survive among a Zomgame bie apocalypse. There is not much action in the sense of shooting Zombies with the left click of your mouse button. The has a high abstraction level and could even be called a board game in some sense.

You have to manage your group of people and their social relations, so they do not rip themselves apart and also have to survive in the chosen scenario.