Probably PS3 Demon's Souls, in terms of time/skill/effort involved. I didn't savescum to the cloud so it also took me the maximum amount of playthroughs required to see and do everything. A lot of people say Bloodborne but honestly I found it cakewalk after Demon's Souls. A second might be Spec Ops: The Line because of having to beat the game on FUBAR but I'm apparently alone in remembering the game as insanely difficult.Damn dude 118!
What was the hardest one?
In terms of rarity it's apparently either Fall Guys or LittleBigPlanet 3. Which, yes, FG took a while and LBP3 required some creativity, but I wouldn't describe either as hard. Just Cause 2 is apparently also 'ultra rare' but that's just grindy (and a lot of fun).
Hollow Knight and Celeste have some truly brutal trophies but a lot of people either glitch through the pantheons of HK or activate the 'assist' in Celeste so the Plats aren't as rare as the difficulty merits.