I believe the games which are openeded and only have whatever goals you set yourself, such as Paradox Map games or Cities Skylines are contenders for rarest achievements as well.
Especially when they have updates and new achievements added as time passes.
For Crusader Kings three as an example, most people can probably found the Dai Viet Empire - But only 0.3% of players has the achievement
I believe the games which are openeded and only have whatever goals you set yourself, such as Paradox Map games or Cities Skylines are contenders for rarest achievements as well.
Especially when they have updates and new achievements added as time passes.
For Crusader Kings three as an example, most people can probably found the Dai Viet Empire - But only 0.3% of players has the achievement