For me Genshin Impact and Hades (including Hades II) are two examples. These games have relatively simple art styles in comparison to e.g. the realistically painted art of certain Blizzard games (WoW, Hearthstone hero portraits), but they are hard to learn even to someone more advanced in art creation.
Genshin has some cool tricks like shell texturing for moss. A very good cell shader and custom vertex normals on the characters, making it a lot of work to recreate. Also it runs on mobile so it is heavily optimised, which also takes more time.
I would put Ori 2 on this list since similar to hades it has a destinct artists style and many 3d layers on top.
But the hardest to recreate by far not just visually but movement wise and ai and procedural animation is Rain world. Just look at the naking of it is insane how much custom tech this game has.