Your rarest steam achievement.

Garrys mod - Mega upload (1.3%)

Ace of Spades - Rocket Man (0.5%)

Probably because once people tried playing it they immediately realized the game was shit and stopped.

Mine is Unstoppable on Torchlight 2 lol (0.3%)

Master Chef On Rising Storm with 0.09%

Spiral Knights - Master Miner, 0.13% <--really a cheap one since it is impossible for people to get this now AFAIK.

"Cherry Ride" in APB Reloaded at 0.31%

Preorder Achievement of Age of Empires 2.

Enemy of the Family (0.79%) on BioShock 2, earned for getting A ranks in all the protector trials. In fact, all of my rarest achievements seem to be from BS2. Probably because I seem to be one of the extremely few folks who actually really really like that game, sadface.

Well I think I will win with this achievement:

Ace of Spades: Undelivered Message Report , just 0.03% players got it ... :)

Best achievement ever ^_^

Btw: You can check it in my profile

Eternal Mom, Binding of Isaac. 0.57%

Explorer achievement from Pillars of Eternity.... apparently only 0.28% of players have got it.

Master Marksman from The Witcher 3: Wild Hunt. 0.18% of players have this achievement.

Wrecking Wilson's Garden from Awesomenauts (0.28%) but I'm not really proud of that since it was an easy achievement.

I'm more proud of 100 Thumbs from Garry's Mod (1.4%) :D

Meet and greet 0.64% - Arma3
Virtual Shooter 0.71% - Arma3
Black Widow 0.77% - Crusader Kings 2
Virtual Pilot 0.98 - Arma3
Tea bagger 1% - Mount your friends
Chew on this 1% - Viscera Cleanup Detail

My rarest achievement on Steam is playing a game.

in general, if you want to know the rarest achievement in all of steam. it probably would be the unattainable achievement from Hate Plus called "Level Four Revive Materia". In fact, the only person that has it, is the game creator herself.
you can read more about it on here. people got so upset, they created wild conspiracy theories for months, banded together to raise 10k and eventually even made a unofficial patch. crazy stuff.
http://kotaku.com/the-steam-achievement-that-nobody-unlocked-1610073943

Synergy "Rebel Rusher" (0.08%)

Hellfire Shells in PinballFX2 .76%

GTA V Career Criminal Attain 100% Game Completion. 0.5% Not very hard so propably wont be my rarest achievement after some time(other players get it)
GTA V Close Shave Complete all Under the Bridge and Knife Flight challenges. 0.5% Not very hard so propably wont be my rarest achievement after some time(other players get it)
CIV 5 Surviving the Marquesas Win as Hiva 0.6% Also not very hard achievement
Risen 2 Mr Industrious Completed 250 quests 0.7% Probably my rarest achievement in future because some Risen 2 achievements are bugged and im 99.9% sure dont get fixed.
Risen 2 Bane of the Beasts Killed 500 monsters 0.7% Probably my rarest achievement in future because some Risen 2 achievements are bugged and im 99.9% sure dont get fixed.

Pixel Piracy - Mr. Croft I presume? (0,26%, likely not many players played since introduction of treasures)
Europa Universalis IV - That's a Silk Road (0,45%)

Mine is from Spiral Knights -- Master Miner 0.13%

"Jerky vendor may cry": sell a foil card for two dollars just before it's price raises at least twice xD

mien is "fastest gun in the west" from PAYDAY 2 with 0,45%

http://steamcommunity.com/sharedfiles/filedetails/?id=448293225 - Clicker Heroes :D