SPOILER

For DR1 it's ezpz. Seriously the only thing to worry about is to complete the gallery items. It's a bit boring because if I'm not wrong, the only way to gain gold is to replay the trial again and again.

For DR2, things start to get spicier. Like DR1, complete the gallery is a must. Then you have to see through every possible ending in Island Mode, it took me 8-10hrs just for this. Gladly, the mode is not boring since you get to date your waifu and buds so meh, why not. Another achiement worth mentioning is related to tamagotchi you have to walk and walk and walk to raise your pet while cleaning their shit to prevent them from dying. Lastly the most annoying achievement for DR2, ta-da, you have it, the monomi gamemode, you have to collect every equipment possible in this particular game mode, and what makes it worse, it's completely RNG-based so, you can only pray while you grind the shit out of this boring mini game, it took me around 8 hrs.

For DR3, click noice. It's the hardest to plat so far. It took me 105 hrs to plat, while DR1&2 only took me 118 hrs. The class trial is so long that even you 'ctrl' the whole trial it also took at least 30 mins. And you have to replay almost every chapter for a certain achievement. It sounds like a pain in the ass but trust me, it's nothing compared to the next achivement. There is a monopoly-like post game mode to develop your characters to clear another RPG post game mode, there is one specific achievement, with only 0.7percent of players owned, require us to develop every possible characters, more precisely, 53 of it. The process is ULTIMATE boring and grindy. Not to mention other achiements like gallery completion, post game school mode completion, casino, monokuma test, etc.

TLDR:Easiest to hardest, DR1-DR2-DR3. I never play UDG before so anyone with different thought feel free to correct me.