My overall completion percentage is my most important stat and the most effective way to increase it is by knocking these out or at minimum get the most achievements I can from the impossible ones.

LIKELY TO 100%
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Al Emmo and the Lost Dutchman's Mine
The Banner Saga
Dead Age
Doctor Who: The Adventure Games
F.E.A.R. 3
Ghostship Chronicles
Idling to Rule the Gods
KHOLAT
LIMBO
The Misadventures of P.B. Winterbottom
Painkiller: Hell & Damnation
Portal 2
Serious Sam Double D XXL
Shadowgate
Styx: Master of Shadows
Think To Die 2
Trine 2
World of Goo
Year Walk

UNLIKELY TO 100%
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My biggest hangups for each are in parenthesis

Aliens vs. Predator (Multiplayer junk)
Angry Video Game Nerd Adventures (Must finish the hardest mode with no deaths)
Bejeweled 3 (Ice columns mode, mostly)
Braid (Speed run that I'm uninterested in)
Dead Rising (The two survival mode achievements are insane)
Deadlight (Nightmare mode is too hard for me)
Death Goat (1,000,000 kills... NO THANKS)
Double Dragon Trilogy (Finishing DD 2+3 on hard mode)
Fork Parker's Holiday Profit Hike (No death run, collect everything run and speedrun)
Grand Theft Auto V (Gonna finish the single player stuff someday but no thanks to Online)
Hard Reset (Couldn't get into this one)
Hatred (Hardest difficulty is ridiculous)
Hitman Absolution (Servers for Contracts mode shut down)
Homefront (Multiplayer junk)
Kan & Lynch 2: Dog Days (Mostly Multiplayer junk)
Lost Lands: A Hidden Object Adventure (Two broken achievements, dev won't fix)
Mother Russia Bleeds (Very hard MP achievements with no online co-op)
Ninja Reflex: Steamworks Edition (Ridiculous MP achievements with no online co-op)
Outlast (Insane difficulty is too much for me)
Outlast 2 (Insane difficulty using only one battery is ridiculous)
RAGE (Played without a guide, don't feel like replaying. Might revisit.)
Serious Sam HD: The Second Encounter (Stupid number of grindy MP achievements)
Super Meat Boy (I don't need to say anything here.)
Tap Adventure: Time Travel (3-4 broken achievements dev won't fix)
Titan Quest Anniversary Edition (Beat Legendary with no deaths and grindy stuff)
Titan Souls (Didn't interest me much, might revisit)
What's Under Your Blanket 2 (All MP achievements broken)
The Witcher 2: Assassins of Kings (Very long game and requires a no death run)
You Have to Win the Game (Not very good at platformers)
Zuma's Revenge (Iron Frog mode, will likely revisit someday)