Your rarest steam achievement.

Perceptual Filter from Guns of Icarus Online. 0.24% have this.

Actually, for some reason, ALL Top 5 of my rarest achievements are from Guns of Icarus Online...

Master Miner in Spiral Knights (0.13%)

Trash Magnate
Sell 50,000 items
0.24% of players

Bolt's Master: From How To Survive, pretty surprised to be honest, is an easy achievement! I guess people didn't liked the game and not played it?

Apparently, Torchlight 2's Butterfly. Interesting, I always thought some of my super meat boy achievements would be rarest.

Blacklight retribution Mechanic only 0.65% people have it, it used to be Batman arkham origin Legend with 0.3% untill a bunch of people got it :(

Genocide from Hammerwatch. 0.17% have it.

Beating Hotline Miami all stages on A+ - 1.6% and getting all achievements - 1.2%.

Some Room Left in Hell ---- 0.01%
We got this ------ 0.01%
From the game: No more room in hell.

I think that i win =P

Mine ist Perfect Knight from Batman Arkham Asylum (1.3%), you have to do everything in the game for it, which isnt that bad, because the game is awesome

Fight for Survival - Blazblue Continuum Shift Extend (0.55%)

Surviving Domesday from Civ5, 0.25% according to Steam.

Dishonored - Rare Collector Yes, that's the name :) - 0.34%

Capture King in Ravaged Zombie Apocalypse - 0,46% have it. I am the boss.

On steam my rarest achievement is Eternal God in The Binding of Isaac with 0.12% of players having it;
on Astats it's Expert of Revival in Orion: Prelude with 0.02% of players having it.

However, the former it's a new achievement of an old game, so there are many players that never tried it, not knowing or not playing anymore; the latter is heavily influenced by being registered on Astats, so I don't know how much is reliable... but wouldn't be surprised if many steam users actually cheated it, it's a real grind, like many achievements in that game.

EDIT: Oh, I thought the question was about our own achievements...
: P

I would search the rarest among free-to-play games, because they have tons of players and the rarity ratio is calculated by achievers/players, so it's easy to have a low number, that is high rarity.

UPDATE: I have searched for the only free-to-play game with many achievements I have played, Archeblade, and I have found that on Astats the rarest achievements I have, The Legend and God of Undersiege, have been unlocked by 0.004% of players.

It seems I was wrong, these are probably my rarest achievements on Astats.
XD

THE DARK SOUL - 0.70%

Plague Inc.
Complete Santa's Little Helper
Score 3 biohazards in the Santa's Little Helper scenario
1.0%

Connoisseur in Civ 5, rate a mod, at 1.1%

This was using the old mod system, the one before they added workshop support. It's now a removed achievement so you can't get it outside of cheating. I guess very few were rating mods before the workshop.

Xenon Wizard from Pinball Arcade at 0.11%.

"Planetary Annihilation" Panopticon 0,35%

Killing floor:
Fire and Forget
Kill a Halloween Fleshpound and Scrake in the same explosion with the Seal Squeal Harpoon Bomber or the LAW
0.89% meh :I

Исаак
http://steamcommunity.com/id/ledwardelric

Blade Symphony Tutorial Achievement - 0,07%. Its hardly Buggy :D

So far, Uncle T-Bag (2.9%) from Postal 2... such a fun game :)

Mine's Ghost from Dishonored (2%)