The WC, and One faith Achievements are fine actually with some interesting choices you have to make along the way.
The Mary of Lotharinga achievement is rather bad as it requires you to disinherit until you get a daughter you can name.
The forever Golden achievement use to be bad as you would often have to release a colonial nation to stop them from promoting develop so you could complete a mission.
Honor mention could be those that require you to have all of X type of provinces in the world, as they really just function as a WC lite.
Marie of Lothringia also works with a consort, it only looks bad on paper but I got it 3x in a row in 3 consecutive Burgundy runs. Just disinherit your heir when your ruler is 60 and you have a Queen named Marie, or if you want to show who's boss you do it with THE Marie of
Lothringia Burgundy
As for most boring one, I dont see how anyone could enjoy one faithing, I abandoned that tedium of a campaign so often it actually drains my will to play for weeks
Carthago Delenda est has to be my most hated completed achievement but only because I was too stupid to think about declaring for w/e county to avoid Call for Pizza
Honorable mentions go to ALL the timed achieves, I hate those with a passion, attempted them once, never again, I rather do Rajput Reich.
Cherry Picking, yeah dont actively go for it, just try and play a lot of Japan and you ll eventually get it.