Steam

Go Outside from Stanley Parable without cheating. Simply don't play the game for 5 years.

Clean hands and Ghost for Dishonored. Never really knew for sure if I had alerted someone or someone I knocked out got eaten by rats until I got the end summary of the chapter, which would lead to me playing the entire chapter again.

I didn't attempt that for Dishonored 2 and just went chaos mode.

Probably 'To Live Forever' in No Man's Sky. Once you get to a certain point, it's almost impossible to die to normal circumstances, but up until that point random crap would kill you pretty easy. Also bugs. Especially with the Space Anomaly.

I ended up doing the 3 hardest achievements all at once in that game. (After double checking it was just the 2 permadeath and survival ones I did together but ended up using my permadeath save to do the "The Sentinel" achievement.)

Also, if you plan to do this this achievement. It's the easiest to just do the main storyline.

Probably my proudest is Crypt of the Necrodancer's Vow Down. The game is already very difficult, but Vow Down requires beating the game in all-zones mode (rogue-like mode rather than the rogue-lite campaign) with one of the more difficult new-game+ characters, Monk. The basic game is a turn-based, grid-based RPG, with the gimmick being that you have to preform all actions to the beat, while the monsters have their turn between beats. Vow Down adds the catch that any gold, be it dropped by monsters or found in the world, kills you instantly and ends your run. Even ignoring the inherant urge to pick up dropped loot, this means you have to carefully position enemies as you kill them so as not to box yourself in with deadly loot, all while only having a fraction of a second to think per turn.

This game also has what is probably among the most difficult (while still being fair) achivememts on Steam: Beating the game with the final New Game+ character, Coda, which has the debuffs of every other character combined. According to Steam, 0.3% of players have this achivement, meaning roughly 0.3% of players have cheated in achievements.

Little Rocket Man from Half Life 2 ep2.

The whole achievement is just so damn stupid lol, I love it! Closing the hatch on that fucker was WAY more satisfying than it had any right to be.

Lazy Bastard in Factorio. Basically automate as much as possible as soon as possible. I recommend everyone go for it to truly appreciate the nature of the game.

The Hammered achievement for Divinity Original Sin 2 which you achieve by defeating an endgame boss in your first encounter with them in act 1. I used a very cheesy method where I chain teleported the boss and her minions on to a rock in the middle of the sea. And then my archer who was on a cliff type thing on the other side of the beach slowly shot them to death. It was very funny!

Maybe this one. Many many hours. So much grinding. Like 100 blitzball wins in a row. Sphere grid completely filled in 7 times. Completely unnecessary, I don't recommend pursuing this achievement.

I think the ones I’m most proud of are beating Journey of the Prairie King in Stardew Valley and the 100% achievements in Spelunky HD and Spelunky 2. Grinded several hundred hours for those

For me, I'd say it's a close call between GTAIV: Ballad of Gay Tony's "Gold Star" achievement for 100%-ing all the DLC missions, or Deus Ex: Human Revolution's "Factory Zero" achievement for completing the Missing Link DLC without using any praxis upgrades or weapons (This one was on Xbox 360, but it's in my steam backlog too!).

A Wound for Every Bullet achievement from Iron Grip: Warlord. This game is obscure and there's barely any achievement guide. I had to figure out how to get this difficult achievement on my own.

I beat HuniePop on hard mode. Does that count?

"Worthy" from Distance.

It is still my rarest achievement, with only 0.60% of players having earned it. A lot of the achievements in my rarest achievement showcase were added to popular games years after the peak of the game's popularity, but were actually quite easy to get. "Worthy" required beating a large level with various unique challenges, and even with checkpoints present in the game, it would often take me hours to reach the next checkpoint at certain parts. I had to follow along with this video at some parts to make sure I was getting through the level correctly, especially the segment from 5:40 to 7:57 which took me about 4 hours to get through successfully.

The "Go Outside" achievement in the original Stanley Parable

Defiant - Hit the trashcan cop with the can in Half-Life 2. Fuck the system.

Playing through celeste every time I beat a b-side I felt like a real gamer until I saw "25% of users have this achievement"

Doing "CQC FTW" and "Don't Stop Running" at the same time from Resident Evil 1. Also "Ink is for Squids" after failing(and falling) at Lisa fight a run before.

"The Impossible Dream" from Steam World Dig 2 also got my dopamine high.

I'm proud of all my achievements equally because I use SAM to get them.