I have 6:
Spelunky
Outlast 2
Super Meat Boy
Metal Slug 3
Halo MCC
Wolfenstien II
And at least 2 more are in games on my to-do list.
I don't feel like SMB should be here either, as I found it to be fairly easy.
I also feel like there were other harder achievements I've done, like just beat Shadow Warrior on the hardest difficulty (17.5 TA Ratio):
Just beat Brutal AI a single time on Tumblestone (took like a month of practice because I swear this game was optimized for MnK, not controller):
Or this pair from Lost Planet 2 which took months of grinding AND required you to achieve #1 on a time trial leaderboard (still a 22.2 TA Ratio and the game came out in 2010):