Probably Traditional Player (get 90+ army and navy tradition). The navy part was brutal, even with maxing out bonuses for passive gain.
I have the achievement for playing a full campaign (as well as for getting all techs as Busoga/Buganda/Karagwe, which kind of is the same except for requiring bad starts) but didn't find them boring at all. I got the former in my first game and felt I had plenty to do still by that point - it was in fact not even the last achievement I got that round since it triggers Jan 1st and the game only ends Dec 31st the same year.
I can see why it would be dull to play a campaign for no other reason than to reach the end, but with enough other goals it's not really a problem.
The tradition one I found easy when I ran a campaign where I deliberately took some bad idea groups. Two Sicilies with Maritime, Naval, Admin, and aristocratic ideas already has the equilibrium at 80. Your trade fleet easily puts that up to 100. I actually had to grind up army tradition to finish it off.
Carthage Delenda Estewas far worse in that campaign (I ended up taking all the land and at the end releasing a nation with just those provinces to sit on. I like the flavour of the idea but it is tedious I would reduce it to having every province scorched.
The play to 1821 and Poland Can Into Space (which I did at the same time) were terrible for waiting with nothing at the end. I ended up just alt-tabbing for like five minutes then checking to see nothing horrible happened before alt-tabbing again. WC type stuff may be tedious but it is something to do. The “and embrace all institutions” get an honourable mention here for being similar but not as long (the Inca one “yay I achieved my goal, now let’s sit around for fifty years on speed 5”), thank god they don’t require the new institution, yet.
However I think Die, Please Die takes the cake. Something you would only do specifically to get the achievement (you would almost certainly have such a leader killed off or abdicate long before 70). Something that is heavily luck as few leaders will live that long. And something without any sense of achievement because it is shitty. I ended up just running a game, starting with an eligible ruler, in the background and reloading an older save every time my ruler died.
