The amount of encyclopedic information I have acquired about so many games is both my pride and shame. I try to 100% every game that has achievements/secrets, or at least get the most difficult ones. I start games on hard and git gud or give up because I figure if there is an achievement for it, better not waste a run on normal so I don’t have to replay it more than needed. My global Steam achievement rate is over 50%, weighed down by dozens of games I never play. Sometimes I give up on those which require more time and proficiency than I am able to develop as an employed adult, or if I lose interest in the game, but usually having uncompleted achievements draws me back in at some point, even for old games.
One of the most brutal was finishing BG2 Steam edition on Legacy of Bhall difficulty without cheating. That is twice the max difficulty of the original version. Everything has double max hp, and the only way to play is kiting mobs for hours, reloading an insane number of times, knowing every secret and shortcut and going straight for the most powerful gear and easy xp.
I 100%ed DS1 and 2, but gimmick Souls runs are beyond me. Gorilla has my utmost respect.
Some achievements I still lack are the raid bosses in the Borderlands games because they require friends who care as much about the game to put the hours in.