Gen 2's level curve is atrocious. You're pretty much nerfed and weakened and have to do a lot of grinding or focus exclusively on heavy hitters to get stuff done easily. I still find GS to be fairly fun games that are my favorite of the GB-GBC ones, but they definitely are rough and Crystal doesn't add much to be excited over imo. Wouldn't really call that hard, more or less "Weirdly balanced". On the opposite side of things the kanto part of the game is broken in half and nobody stands a chance against you (they didn't really fix this in HGSS either, unless you rematched them in the dojo which finally gave them good teams). To be blunt, most of the hardest games in the franchise are honestly due to bad design decisions in some way or another, with Gen 2 being due to the level curve, and I remember getting wrecked hard by Bugsy and Whitney, but literally nobody else as I was able to steamroll them due to the options opening up after Goldenrod. (You also can get a machop in a trade to steamroll whitney if you notice that ahead of time, so even she isn't safe)
The only other game I could really think of that's shockingly hard, but to the point of insanity and utter hair-pulling (that aren't the stadium games), would be Fire Red and Leaf Green. Somehow, they took the normally super easy Kanto, and tried to toughen it up a bit... Only to mess things up horribly. There's no move relearner (just like the originals), meaning that the moves you put on your pokemon are the ones you're stuck with, so if you end up needing Psychic but all the pokemon you have can't learn it, better use your sole TM to help your team! But unlike RBY, the AI is actually good due to being based off RS, so they'll kick your ass and spam status conditions to hell and back, along with being at pretty high levels compared to the general levels scale. Replaying that one and beating the Round 2 E4 was incredibly infuriating, but I can't help but admit some parts were pretty edge-of-my-seat intense and I liked some of it. Still lame that you couldn't relearn moves though, and I think that could have done with tweaking. (LGPE I feel finally nails the difficulty curve as doing the gym badges out of order makes things tougher and allows you to make it more fun that way) BW1 also has a nasty difficulty spike at the Elite Four, but otherwise that one's fairly standard
I definitely do agree with the consensus on ORAS/XY being braindead easy though, especially the former. It's night and day compared to the originals, which had a fine level curve and the only gripe being the limited use TMs, but ORAS basically overpowers you with little options to turn it off (EXP Share isn't even the problem here) and doesn't adjust the older trainers to take that in mind. They also removed tons of stuff like the challenge levels of contests, several side activities and minigames too. I could probably beat ORAS blindfolded, which I can't really say about any other Pokemon game. XY is pretty easy too but that's mostly due to the EXP share as the game's scaling is pretty fine without it on, since they made the game seemingly with the way it was done with Gen 5 in mind.
Gens 7 is much better when it comes to a reasonable challenge I feel: Base SM is the perfect level of difficulty that isn't too weird or insane, but not mash A either. Pretty much how I wanted the series to go forward difficulty wise, but USUM is a weird oddball in that it's suddenly the old school difficulty with a lot of cheapness thrown in. The main boss of the game even outspeeds your entire team and is likely to be many levels above them too, which is just BS and makes the game not that great. I had to transfer a Level 100 Articuno just to beat him and he even nearly killed it in one hit due to all the stat boosts the game provided him! So if you're wanting a newer game that's harder... That's one option, tho not a great one if you ask me.
SWS is pretty fair like Gen 7, but scales back a tad. It's still reasonable though, limiting yourself to only mons caught in-game and not using candies. The candies let you break the game in half, so it really just lets you get the best of both worlds. I only used candies on my starter, and while he was higher than anyone else, the rest of my team would still have a fair challenge going up against the rivals and gym leaders. I even lost the battle against the champion since somehow not even my candy'd starter could survive a hit from his ace mon, which pleasantly surprised me. I think that one's fine if you know what you're doing.
But yeah, Pokemon difficulty is... weird. It was definitely tougher in some ways back in the 2D days, but some of that felt unintentional or relied on cheapness. I feel RSE, Platinum, and BW2 are the main games that perfect the difficulty curve and manage to still make you think. SM would be the closest 3D Pokemon came to that difficulty curve, with USUM being a (bad) second, and SWS being third while all of Gen 6 is dead last.
