Imo, motifs collecting is the most difficult to get.
Searching in guild traders is a nightmare with the pages and pages to scroll through, not to mention the money you have to accumulate to purchase them.
Farming means patience testing given the rare drop rates.
Farming Motif in vet Mazzatun hard mode means everyone need a mic to coordinate on last boss, which I don't use.
Alvari motif need to farm insane amount of alliance points.
I think it's the most difficult tbh.
Is emperor easy? It's real competition and the amount of time need to be spent in Cyrodiil is insane. Besides you need a team and people skills I reckon.
Everything else is fairly straightforward. Emperor, you have to PvP like it's your job through the whole campaign, and be good enough to farm AP efficiently, and usually have group support through the whole thing. And you're fighting humans, not programmed encounters.
Flawless Conqueror is simply a matter of running the arena enough times to have it down. Master Angler isn't hard it's just long and tedious. Motifs is just gold farming ... and drop sets can't be called hard because it's random - kid in my guild got a sharpened lightning destro on his first VMA clear, other people have run it 500 times without a sharp destro ever.
Also, Dro'Mathra Destroyer should be on this list.
Of the options present, probably Flawless Conqueror. But among all titles / achievements I would say it definitely has to be Dro-m'Athra Destroyer (I realize now I probably just should have checked "Other").
Getting all the monster collectibles. Not as hard now with OneT since this removed the 5-level restriction. Still harder than Master Angler since the drop rate seems lower than fish. Unfortunately, the achievement grants dyes but no title.
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Depends on your definition of "difficult". Master Angler might take longer total than Flawless Conqueror, but it requires zero skill and is just tedious.
FC on the other hand requires tons of skill. In that regard, Dro-m'Athra Destroyer would be even further up the list.
I have emperor and flawless and all of the sets, recipes and motifs I want, none of those were that hard to get, I would go for fishing. I am not sure I will ever get that, too boring, I start falling asleep after 5 minutes of it...
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These so called 'achievements' are just a matter of doing some thing enough until you hit an arbitrary number.
For the most part I ignore them. I play normally and some achievements get done. Others don't and I rarely go out of my way to finish them. I have for a few dyes and for crafting.
I'll make a special point about motifs.
Prior to master writs I learned the motifs for the styles I liked and just ignored the rest. A lot of them really aren't to my tastes and it's the earlier ones I like. I am making some effort to collect the others now but that's just an irritating time / gold drain which is sucking all the fun out of the game.
emperor isn't that hard to get- relatively speaking. it just takes alot of free time. and no job, active social life and to neglect your pets for a week. the others like angler require patience- to me thats harder.
Dye Collection is technically most correct since it requires several of the hardest on your list (Master Angler, Emperor) and one you didn't (Grand Overlord). You cannot have a complete dye collection without these three, and they are some of the hardest things to get in the game.
With that said, getting the desired trait on a particular item can actually be literally impossible because that's how RNG drop chance works, but on average it's going to be the dye collection.
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I think it depends greatly on how you define difficult. Many of those achievements are not traditionally difficult but are time consuming. Master Angler is extremely time consuming but pressing 2 buttons back and forth forever is not difficult. Also Dye Collection is by definition strictly more demanding than something like Emperor or Master Angler as a full dye collection would contain both of those achievements as well as a grand overlord.
The only truly difficult achievement on that list is Flawless Conqueror as that's the only one that requires player skill to achieve. One might make an argument that Emperor can take some skill as well but even if that's the case, it gets trumped by dye collector for the reasons listed above.
I chose to look at it more like which achievement is the most frustrating to achieve and my choice is based on that.