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.

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

0.4%

Not as rare as some others, but takes a little while to achieve:

Rarest

Proudest (Love this game, highly recommend)

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

Somehow I'm among the 6.1% of Stanley Parable players with this one.

Terraria, 0.4%

Close to my rarest, but very fun and surprisingly strong! From Victoria 3

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

0.2% in CK3.

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 only have 20 Civ 6 Achievements, and one this one is also my rarest steam achievement somehow. Pretty sure it's just a thing people don't try, rather than being hard.

Not strictly my rarest

But I remember taking a good while to actually achieve it (FTL)

Napoleon Total War. Not sure if I was achievement hunting or not but it sounds fun.

Watch_Dogs, 0.2% achievement

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.)

My two rarest aren't that rare. I'm not an achievement hunter.

GTFO 0.2%

After that I’ve just got a bunch of ones from Risk of Rain 2 from just unlocking everything. Pretty time consuming, but very much a brain off game.

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

I am kinda saddened that it's such a mundane achievement but I'm also baffled because I didn't even go out of my way to get this one I just played normally