Heh. Difficult Pokemon games. Alright, I'll play along.
I'm going to sound terrible, but...Red/Blue/Yellow. Seriously, go back and play them now, all these years later, coming off the 3DS titles...or even the DS titles. They've not aged well at all. Red/Blue slightly more than Yellow, given that Yellow gives you all three starters...although you also start with Pikachu, so if you don't know where everything is, or what learns what at what level, the first Gym is going to stonewall (haha) you.
Pretty much every Pokemon title from Gen III onwards spoils you shamelessly with significantly enhanced gameplay mechanics, a lot of hand-holding and signposts about where you need to go next, a wider variety of Pokemon along the routes, etc. Recently it's also been forcing gift Pokemon onto you and shoving you down linear routes to the next challenge with all the subtlety of a speeding locomotive. You can't not know what you're doing in new Pokemon titles, with everyone going out of their way to tell you...and roadblocks reminding you if you forget.
Pokemon is designed to be played by everyone, so it has to be easy enough that children can play it...but kids these days, they're spoiled. Gaming has changed a lot since the 90s, and Pokemon has become substantially more accessible (read: easier) to accomodate this. This is by no means a bad thing. But it's VERY jarring when you go back to the earlier titles and the kid gloves come right off. Easy enough to play still...if you know what you're doing. I would argue that you don't really need to know what you're doing to play later Pokemon titles and have an easy time of it still.
This is not to say that RBY are inherently different for lacking these signposts, because they're not; you can still blaze through the game if you grind enough, etc. But it's going to feel like even more of a trial doing it, and the limited systems are pretty jarring when you've been playing 3DS games for years and you're used to being treated more gently. RBY are Pokemon without the padding. Still easy, but VERY rough around the edges.
As time goes by, unless we get a weird return to limited systems and a complete lack of direction in Pokemon games, RBY are only going to seem harder for that, I think...which will probably only serve to have people begging even harder for Kanto remakes, not realising Kanto remakes would just adopt the systems Pokemon has now.
