Your rarest steam achievement.

Empress in Mount and Blade: Warband (0.5% ).
It really just happened, I never hunt achievement.

Monaco : Play It Again Sam @ 0.52%
Train Fever : Industrialist @ 0.52%

Lead and Gold - Gangs of the Wild West
Natural Born Leader - 0.65%
"During a multiplayer round, gangmates kill 10 enemies tagged by you."

So Lonely from Mount Your Friends, 0.75% earned. Sums up my life perfectly.

I never notice achievements, but I did get one for getting an 11/10 score for a game on Game Dev Tycoon. I've never gotten even a ten on a game since, so it shouldn't be too common.

Dead Rising 2 - "He hasn't covered wars". 0.31% :D

Catan, On Principle achievement, 0.07%. (Which is quite funny, considering it is a very easy achievement; just nobody plays that POS adaptation.)

Long Distance Runaround - 0.14%
Truly a Stunning... - 0.14%

both from Carmageddon Reincarnation

20 car pile-up (1,1% of players have it). No boosting, got it the fun way. :)

An Army of Four (XCOM:EU) @ 0.64%

.97 Hoarder from Unturned

Samurai Delicatessen from civ 5, only 0.38% have it

Some Room Left in Hell - No More Room in Hell (0.01%)

Eternal God - The Binding of Isaac 0,13%

My rarest is British Invasion in Civ V, only 0.35% of players have it (albeit being a really easy achievement to get).
But my proudest one is The Dark Soul in Dark Souls 1, 0.73% of players.

15 mln Mastery Points in One Finger Death Punch, 0.38 % of players

Monday Night Combat - Uber Streak (Score a 25 Kill Streak) 0.07%

The end from Super Puzzle Plataformer. Only 0.75% people have it.

Goat Simulator - Love or Hate? (0.49% according to Steam.)

according to my steam showcase..

mount and blade warband: empress 0.52% manifest destiny 0.71% Girl power 0.72%
sniper elite 3: living in the shadows 0.71% Shoot to thrill 0.76%
Universe Sandbox: Startup Kilo 0.78%

Dorian Grey from Guns and Robots - 0.06% of players have this

They don't pay us enough- PAYDAY 2 (1% of all players)

cash for trash, TL1. .3% of the user base.

Mine is "Master Miner" in Spiral Knights with 0,13%, which is doomed to remain this rare since it cannot be obtained anymore =)