Steam doesnt track completed games if theyre free for some reason, so heres my manual list
I used to maintain these to keep them all 100%ed if new achievements were added but the list is getting too long for that XD
some of my favourites will be maintained, otherwise enjoy this snapshot in time

(including games i 100%ed on xbox)
Papers Please
Bad North
Journey to the Savage Planet
My Friend Pedro
Shapez
Trailmakers
Slime Rancher
People Playground
When the Darkness Comes
Superflight
Hexcells
Hexcells Infinite
Hexcells Plus
Portal
Superliminal
Clustertruck
Helltaker
Wallpaper Engine
Risk of Rain 2

Deep Rock Galactic
Steam Engine Simulator
THE BUTTON
Grapple
I commissioned some bees 0
Femboy Aim Trainer
Furry Femboys
Getting Over It with Bennett Foddy
Roboquest
Viewfinder
Echo Point Nova

A Game about Digging a Hole
Gray Platformer
OrbitXL
Achievement Clicker
Firewatch
Escape from Boykisser
Deepest Sword
100 Hidden Cats
Cats Hidden in Paris
I Commissioned Some Bees 1
Pirate Fighting Simulator
Unsorted Horror
Tower Wizard
Manifold Garden
Achievement Clicker 2018
Achievement Clicker 2019
Stone Simulator
Fill Up The Hole
The Exit 8
PEAK
QUICKERFLAK
Digseum
Nodebuster
Warhammer: Vermintide 2
Magic Archery
The Witness
Tametsi
A Difficult Game About Climbing
Leaf it Alone

Far Fishing