It's not an achievement, but this is pretty rare

Video game news oriented community. No NanoUFO is not a bot :)
Posts.
Comments.
My goal is just to have a community where people can go and see what new game news is out for the day and comment on it.
Other communities:
It's not an achievement, but this is pretty rare

I don't meet many people like you anymore.

damn, y'all, how big were the dinos back then ?
Big enough to screw up the platters on my hard drive when they stomped by!
You had to walk into the computer to catch the bugs.
'cause we're dying of old age? :P
I was very anti steam when it first came around and only got an account once CS forced me to to continue to play. My join date is the beginning of 05 so you must have been a super early adopter!
What about the inverse?
I got Satisfactory before the 1.0 release. I played most of the game before achievements were added. Now the achievement for "reaching the credits" is achieved but "finish the tutorial" is still waiting to be done.
Wow that's hard to figure out, I went looking for games I had the most achievement completion and tried to find which one had achievements most people had that I didn't:
Not as rare as some others, but takes a little while to achieve:

Still working on Super Go Outside
Napoleon Total War. Not sure if I was achievement hunting or not but it sounds fun.

0.4% Pillars of Eternity 1: No Rest for the Pro. Complete the game with fewer than 10 rests.

Definitely not as rare as most of y'all and probably only as rare as it is because of how many millions of people play stardew valley without ever even trying the arcade machines.
Back when RDR2 was fresh on PC, dinked around once with the death match modes and never again lol.

My rarest achievement is from The Long Dark (Finely Tuned) at 0.1%. But that's because not many people played the last episode when it came out.
My most memorable is from Titanfall 2 (...Becomes the Master) Place in the top 3 of the Gauntlet scoreboard. That one is at 2.9%
One of the rarest achievements I have is not at all difficult. It's not even trying to be. It's on Street Fighter 5 for saving 30 replays. Apparently nobody does that?

I'm this many years old on Steam and don't know how to find my achievements on the mobile app.

Not as low as some other percentages in this thread but I was very proud to beat the last pantheon in Hollow Knight. You have to beat every boss in the game in a row without dying

Close to my rarest, but very fun and surprisingly strong! From Victoria 3 
Not at all an achievement hunter, and Steam is not my default gaming platform...
But I checked my rarest achievements and I've got a 1.4%. Which does not sound much, except it's in fucking Skyrim, and absolutely trivial to get.
OK, there's the catch : it's because this is Skyrim VR. It's the achievement you get just for mining ore and chopping wood.
Looks like there are some relatively easy .3/.4% to get there if you've got a VR headset and some dedication. And manage to keep the game working long enough to get those. That might be the hardest part, because ... Bethesda.
I don't have it on Steam yet, but I have Balatro's Completionist++ on mobile.

I'm working on getting it on Steam too. Once you've done it once, getting it again isn't hard, just time-consuming. It's a 0.2% achievement, I think.
Every device I own eventually becomes a Balatro machine.
Phone? Balatro Machine. Steamdeck? Balatro Machine. Work Laptop I can't install programs on? Oh, look at that. Another Balatro machine.
Ah I see witch it.

Heart of the machine, 0.4%

It's not so rare now; but on the second day Silksong was out, I got the achievement for defeating Phantom when it was still 0.1%
0.2% in CK3.
Thank you for asking!

My rarest achievement (0.3%) and most of the other rare ones were mainly for winning the Godly difficulty of the Neter campaign on From The Depths with my own creations. Each faction was its own achievement.
My second-rarest (0.4%) and proudest achievement is my tenth dan black belt in Toribash. I'm really close to custom belt but I haven't played in like 20 years. Oh my god.

Lazy Bastard, Getting On Track, Raining Bullets, TIS-100, and a couple Noita ones
Divinity: Original Sin 2
A friend and I managed to finish the honour run in less then 10 tries. It was awesome at the end. I played as Sebille and he as the red prince.

Oh Nosgoth, we hardly knew ye...

Watch_Dogs, 0.2% achievement

Not strictly my rarest

But I remember taking a good while to actually achieve it (FTL)
Is there an easy way to find out without using third party websites?
Set the "showcase" at the top of your profile to "Rarest Achievement Showcase". I'm not sure how else to see it natively.
In your steam profile settings there's a tab named Showcase Manager in which you can choose to display the Rarest Achievement Showcase on your profile page, it will show your rarest achievements on your profile

Shadows of Doubt. These achievements are not rare because they are difficult, but because they are tedious. Most of the ones listed here are just repeatedly eating a food item, which costs almost nothing (1200 credits for solving a typical case, 700 for an easy case; 12 credits to buy a Croque Monsieur, the most expensive food in the game.)
Rat Catcher: Solve a murder while your PoV is at the ground, but you can crawl up walls, and also nobody will talk to you. This achievement is rare mostly because this game mode was added a few years after this game peaked in popularity.
Spare No One: KO everyone in town, in a game where you are discouraged from using violence. Finding a katana and knocking out everyone you see on the street is easy enough. The real challenge here is keeping track of everyone. Some NPCs glitch out and get stuck in their houses, never go outside, and never come to work. You'll have to identify them, find out where they live, and hunt them down. Some NPCs have accented characters in their names, making them difficult to search for on the computers in game, which do not allow you to type such characters.
If you make a case board to keep track of everyone, the case board takes exponentially increasing time to open for each person you pin onto it. By the end, it was taking me about 5 seconds each time I opened it. That was in a custom procedurally generated town of the smallest possible size -- it would be worse on any of the stock towns.
Out of the Red: pay off 10,000 to a loan shark in 1,000 credit increments, one payment per day. Raising 10,000 can be done in half a day easily. Paying it off takes for ever. You can't make payments early. Each one-day wait takes a few minutes, since the game has to simulate every NPC in town. And remember to turn off the procedurally generated murders, because otherwise your loan shark might get killed, and you can't make payments any more (it happened to me, and I had to load a save.)