What was the most difficult achievemnt you've earned?

Maybe Seriously for Gears of War 1 was the toughest and longest but I think there were other of them worse than this, but will be worse in the future, maybe Seriously 3.0 if I'll earn it in this life..

the one on rocket league score a goal upside down in the aqua dome arena

According the TrueAchievements it's beating Two Worlds lol

"The Challenge" from The Witness is my go-to answer for this question as well. One thing I love about "The Challenge" is that it still manages to still have a few new or rarely-seen-before tricks up its sleeve, yet it's approachable using similar lines
of thinking that you use throughout the rest of the game- only also with that new constraint of being under a time limit. It really felt like a test of mastery of the game. It also probably took me at least 100 attempts before I got it.

I'd also mention "All the Trimmings" for getting three stars on every kitchen in Overcooked. I did this in local co-op with my s/o, and it was quite a test of our communication skills, planning, and execution, particularly on a couple of the ice stages.
This one is especially memorable for me because it wasn't that feeling of "holy moly, I did it," but that feeling of "holy moly, WE did it" that made it special. (It does only unlock on Player 1's profile, and we did eventually go back and complete it on
both of our profiles.)

The Dark soul from Dark souls 1