Recommend me some hidden gems you've enjoyed

The Council. Calm evening with coco and playing this game was very relaxing :)

The ones I was most surprised by up until now were Intrusion 2 and Spark the Electric Jester.
I'm listing them in my personal curator page here, in case I find more interesting gems in the future: https://store.steampowered.com/curator/31699331-Auricommend/list/14379/

K, I'll start with visual novels for no good reason at all.
428: Shibuya Scramble
I wouldn't call it hidden, but it surely isn't as popular as other Spike Chunsoft VN titles. I guess sales-wise it flopped pretty hard, but I enjoyed the hell out of it. I mean, you don't get a live-action VNs that often. And it's really thrilling and funny, to boot.
https://store.steampowered.com/app/648580/428_Shibuya_Scramble/

Analogue: A Hate Story & Hate Plus
Non-jap VN, pretty weird from gamemechanics' standpoint. You're pretty much deciphering the abandoned spaceship database together with cute-girl-AI (obviously). It's kinda hard to get into since you don't know anything and just forced to read through lives of a couple dozens of Korean peoples (remembering the names is the pain in the ass as always) you know nothing about. Sometimes the stories go full lesbian for no reason (except game designer's interest, but at least it's okay written), sometimes it's kinda disturbing, sometimes it can be straight up boring (depending on how invested you are), but overall it was pretty interesting.
https://store.steampowered.com/sub/32136/

Wonderful Everyday Down The Rabbit's Hole
I haven't played it till the very end yet, but Jesus Christ almighty, is that novel going bonkers pretty fast. If you're ready to watch some (or suffer through) some kinky fetishfest it is. It has... let's just say, things that are not okay. It gets really, really, really dark. "It's my own invention" chapter really... surprised me, to say the least.
https://store.steampowered.com/app/658620/Wonderful_Everyday_Down_the_RabbitHole/

Forget me not: My Organic Garden
It's a clicker that keeps you invested with the story and the concept.
https://store.steampowered.com/app/363980/Forget_Me_Not_My_Organic_Garden/

Masochisia
A genuinely creepy and disturbing experience.
https://store.steampowered.com/app/396310/Masochisia/

Also
Qora https://store.steampowered.com/app/304460/Qora/
Naissance https://store.steampowered.com/app/265690/NaissanceE/
Kero Blaster https://store.steampowered.com/app/292500/Kero_Blaster/
Rollers of the Realm https://store.steampowered.com/app/262470/Rollers_of_the_Realm/
Vangers https://store.steampowered.com/app/264080/Vangers/
Hello Charlotte (you prob should play the free parts by the author first, it's Ep.1 and Delirium - they're not on Steam) https://store.steampowered.com/app/557630/Hello_Charlotte_EP2_Requiem_Aeternam_Deo/
Bad Mojo https://store.steampowered.com/app/255960/Bad_Mojo_Redux/

Tower 57.
Apparently some folks decided The Chaos Engine was long overdue for a spiritual successor, so they decided to build one with more storytelling, more modern gameplay mechanics and more demented character designs.

The Deadly Tower of Monsters is one I really enjoyed but not many people seem to have played. I especially enjoyed what they did with the commentary about 2/3 through the game (don't want to spoil it, but if you get that far I'm sure you'll agree it was a nice twist).

I don't think I saw this one mentioned ever but I liked it so much (walking sim) https://store.steampowered.com/app/265950/Ether_One/
Also https://store.steampowered.com/app/378610/Valley/ was a very nice surprise. The mecanics felt so real and funny I caught myself jumping in some field completely forgetting I was playing a game for over 5 minutes. Just for the fun of jumping around. The game is also very nice too :).

One Finger Death Punch!

CrossCode. And it'll blow your mind I'd say. At least you won't expect it but it will.

ACE Academy is a good and casual visual novel, saying that as someone that isn't too much into it.

Way of the Samurai 4 is a strange game, but a good one at that. You're just thrown in the game's world and you can basically kill anything and anyone, or you can follow through the story to reach one of the 20 or so endings.

Mad Games Tycoon is actually the most detailed and less scripter game dev game on Steam right now.

Empire TV Tycoon is a game where you plan a TV channel's programation and try to build a good reputation for it. Pretty cool concept, nice to play every now and then.

Sol Trader - relationship simulator. If you want to read a lot through dialogues and intrigues, building your character reputation in space and all that, this game is for you. It isn't for everyone though, it can take more than 8 hours to you manage to get into the interesting parts of it.

If you're into thinking and relaxing, I highly recommend Hexcells and Squarecells.

Survivalist,This Strange Realm of Mine, The Last Time, The Adventure of Tree, The First Templar, Armed with Wings, Shadow Ops - Red Mercury, Beep, Ninja Blade, Gun, Sweezy Gunner. Cold Fear, Armed and Dangerous, Poi. I can list more, but unless you are willing to buy a copy from Ebay, there is no other way to play them :)

It really depends of your tastes. But here are few game not so known that I really enjoyed:

Full Bore A mix of metroidvania and puzzle. There is no fighting but this game is one of my favorite. Puzzles are really well done, it's enough fast-paced and you can rewind your actions anytime if I remember right. And the soundtrack is just perfect (watch the trailers).

Prelogate It's not the sexiest of puzzle games, but it's one of these game where you build your own solution and feel clever when you've succeed the level. It reminds me Zachtronics games with a simpler approach.

Expeditions: Conquistador A really well done tactical RPG in an unusual setting. The difficulty is well balanced, there is a tutorial, there are decisions to take, ... : a flawless experience.

Desktop Dungeons it belongs to a specific subgenre of rogue-like (with games like Runestone Keeper, Dungelot : Shattered Lands, ...) and it's the best of its category. Games are short but there are various modes/extras to unlock as you expand your kingdom which make the content of this game huge. The ideal game for coffee break.

Valdis Story: Abyssal City One of the best metroidvania on Steam. I really enjoyed 2 things more than in other metroidvania: It's a maze, I like when you aren't guided by the hand in this kind of game. And the fighting system is great. The story is forgettable, it's the only flaw.

And I saw games you already have but not played which deserve to be mentioned:
Ittle Dew A game where I expected to see more reviews because the art-style is so cute. A short game with Zelda esthetic but it's also a puzzle game. It's short and there are different routes to end it.

Of Orcs And Men One of these games that you will hate or love. The gameplay is average but the story was unexpectedly great. The characters are perfects and there is a lot of dialogs. It's the game before they made Styx series which I loved too but I preferred the story of this one.

Hidden gem means that it's something most people never heard of, right? I see some popular titles here, so I'm not sure anymore.

With my definition in mind, Super Panda Adventures - great metroidvania.

They won't be hidden gems for some people but I highly recommend:
CHUCHEL if you like to laugh a lot and admire fantastic, cute art when playing a game (there's not much in terms of the gameplay but I've had suuuuuch a great time with it anyway)
OneShot if you like to get emotional because of the story. Do yourself a favor and don't read anything about this game to not ruin yourself the experience. The gameplay in OneShot is really unique and soooo effective in it's simplicity.
Hue if you're looking for a puzzle-platformer with a fun and unique gameplay. The best way to describe it is to watch the game's trailer – it explains everything perfectly.
Epistory if you like to type :P It's a typing game but really enjoyable to play once you'll sink into it – not sure about others but I needed some time to start having fun (not much though, just a warning it might look a bit bland at the beginning but later it only gets better). The story might look quite generic but there's a nice surprise later on that will make you love it ;)

Kingdom wasn't very hyped game on release. And the game is just awesome, interesting mix of relax and difficulty, and this marvelous pixel-art.
Eador. Genesis - just hours and hours, some king of nostalgy to HoMM3-4 and Disciples1-2.
The Curious Expedition - interesting idea, combat-system. Fun for several hours, for sure.
Detective Grimoire - great point-and-click(and short, that's a pity), can't say more.

No recommendation atm, just here to appreciate someone else giving a plug for Anima :D

Won the game here and had to 100% the game because it was so much fun.
Was even more surprised when I heard/saw it was only developed by 3 people^^

Incoming Forces(2 games in one,btw)
The Kohan games
GUN
Sacrifice
Gun Metal