4th Gen Difficulty.

D/P/Pt were the harder games. I've always found myself struggling near the end because my team was underleveled, haha. I would always dread meeting Barry, for some reason. He just shows up whenever I want to get to the PokeCenter...!

HG/SS were definitely less of a challenge. I remember back in G/S the Kanto section was just easy to breeze through, and while they somewhat fixed that in HG/SS, my team was always a few levels higher than theirs. I think the only trouble I had was with Clair's Pokemon. Those Dragon-types were so painful to deal with, haha. Oh-- I remember my first battle with Red back when the games were released. It lasted close to an hour! Good times...

I didn't find any of them incredibly difficult.
I remember having some trouble with Platinum's Elite 4, but nothing from HGSS threw me off, mostly because I had a solid team from the 3rd gym onwards.

DP were really hard for me, gardenia's roserade, fantina's mismaguis, candice's... everything (i had torterra) and cynthia's garchomp kept knocking all my pokemon, it was really hard

Platinum was easier, I think it was more balanced, and cynthia's garchomp was 4 levels lower (thankfully)

I'm actually playing righ now Soul Silver for my first time (I played a lot the silver and crystal versions), It's kinda weird haha my pokemon are on lvl 36 before the final gym, that would'nt be enough to knock DPP's sixth gym :P but I guess that's how the game is...

I actually found Diamond, Pearl & Platinum to be somewhat of a challenge, with the Elite Four members having teams of five Pokémon ranging from 50-70.

Diamond and Pearl were mostly easy for me, except for Lucian at the end. I could easily KO all his Pokemon, until his Bronzong. I don't know why, but I didn't have a single Fire type move on my team, and my entire team was underleveled. I kept losing, and the only reason I eventually won was because of a crit. Yay, I guess.

I never played Platinum, but I don't think it would have been too hard for me (after DP was when I got into competitive battling, and actually got somewhat good). Hopefully I would have chosen a Fire type during that run-through.

As for HG/SS, I actually found them to be a breeze, even Whitney. My team was really overleveled, due to my grinding a lot. I think I already had a Feraligatr by the time I battled Miltank, which I guess made it easy.