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.

The "Go Outside" achievement in the original Stanley Parable

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.

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

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.

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!).

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"

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!

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

Playing the game or starting the game achievements

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.