Perfect Games is in between total and percentage on my Showcase, and those of my friends who’ve unlocked the Showcase. (Figuring out which games are Perfect requires memory or looking through your library, though…wish it would actually let you see them…)
10, though with some semi-dupes:
Portal
Portal 2
Deus Ex: Human Revolution - Director’s Cut
Deus Ex: Human Revolution
Deus Ex: Human Revolution - The Missing Link
Fallout: New Vegas
Banished
SOMA
The Swapper
The Vanishing of Ethan Carter
I could swear that I completed Cities: Skylines as well, but I think they added some achievements since I last played. My rarest achievement is “Tough City” in Skylines; of the completed ones, it’s “Tenure” in Banished.
Four.
Just Cause 2
Destroy All Humans! Path of the Furon
Elder Scrolls IV: Oblivion
Far Cry 3
With Just Cause 2 being the hardest to achieve according to TrueAchievements.com. Many more I would have done in my younger days before achievements were a thing, but generally I can’t be arsed to do that much grinding anymore. Too many other games to play and things to do.