Which main series game did you find to be the hardest?

In order:
1. The original Black and White. Why?
-Limited amount of Pokemon. With no prior knowledge you have to learn how each pokemon works. Too many Unova mons evolve at a high level and unless you grind it is impossible to do it pre end game for some of them (Hydreigon) without any trading or so forth.

- The AI and movesets got a bit of change this time around. This not only helps you with having access to newer decent moves but so does the enemy AI. Not to say every battle is difficult, but there is a noticeable change compared to gen 4

-Ghetsis. Having Ghetsis as the final boss gives me such anxiety. The Hydreigon is one thing but dealing with an entire team of bulky mons is absolutely dreadful (in a good way). Not only does his Hydreigon have coverage for any possible type it hits like a truck. The only Pokemon I had to survive that was the Reshiram the game just gave me. And even then, I had to heal.

-The post game levels are outrageous. I guess they were just figuring out the Exp and how to make it work this gen onward because having to face level 60 mons when I'm still mid-late 40's is something else.

2.Ultra Sun and Moon

-Why is this 2nd? You actually have a variety of Pokemon in this game which is what I like about gen 7. You can learn new moves easily with BP and you have better TMs to use over and over. That and the game is very generous with money/healing. But does that make this an easy road? No.

-The totems
All of them in the Ultra Games and OGs are annoying in some way. Dealable, but still. Lurantis? Bring backup to confuse you and have an item to instantly use solar blade. Araquanid? Have Aurora beam for any grass types. Togedemaru? Use spikey shield over and over again. Rhibombee? Use quiver dance and sweep. Have a berry and summon Pelipper to make things harder too. Mimikyu? Don't pay attention, you wasted your Z move. Can't use a priority move like Extreme Speed. Then you have Haunter that will constantly hypnotize you. Say what you want about the game being easy but at least this time around you get screwed if you aren't prepared.

-Ultra Necrozma.
You have to find a way to beat this thing and survive it's attacks at the same time. If you can't stall or take it out quickly you won't survive. Poor Nuzlockers end here.

-The variety of Pokemon.
Because many of the trainers aren't just using Alola mons in the main game, it adds a bit to this. I can remember strictly a few annoying trainers thanks to carrying bulky mons.

3. HeartGold/Soulsilver.

-The level curve is outrageous. Having to face each gym so quickly really messes with the leveling curve. I remember beating Pyrce at level 27. It's so easily to be underleved in the this game and the fact they didn't really change this in the remake makes it worse.

-Your variety of Pokemon are really weak. A lot of Johto Pokemon are super weak and not worth using stat wise. Sure you can like them however you please, but try dealing a Ledian against a Dragonite and see how it goes for you without any proper planning. To this day I stick to only a few specific Johto mons because they're either A. Hard to find, B. Stat wise really weak, or C. Don't learn really good moves.

-End Level trainer buffs require a ton of grinding. Want to survive against the Elite 4? Unless you figure out a good strategy you have to grind. Red? He got a buff so you have to grind. The Kanto Leaders aren't bad as you have trainers to work with to even things out, but the highest levels I had with a full team was around the 60s, all with a fast experience yield. There's no way you can match in Red's levels by that point without some form of grinding. It's especially bad here.

Generation 1 on first playthrough, for the lack of a lot of information that could have been useful, like what the different items do, or what a certain move does (and why I shouldn't replace my Charizard's Flamethrower with Fire Spin... I'm still bitter about that 20 years later) .

Generation 2 improved on that tremendously simply by providing blurbs for items or moves, but unfortunately not where it really matters: in the menu that lets you replace a move when a Pokémon levels up, and if you're abandoning a new move, you're never learning it.

Honestly? Any game set in Jhoto. For some reason I just can't get my team strong enough by the halfway mark to actually take on and defeat the Elite 4 and Champion without having to grind on wild Pokemon for literal days (bearing in mind I challenge every single trainer I can find along the way), and I just don't have the patience for that. To this day I still haven't managed to explore Kanto outside of games set in it to start because of this, and Jhoto remains one of the only two regions in the franchise I haven't entered the hall of fame in.

If you're wondering, the other one is Unova, but that's because I keep flip-flopping on what Starter I want to use and starting over. I've gotten close, but it isn't difficulty that's been stopping me, it's been my inability to choose between Serperior and Samurott. Doesn't help that I got White 2 pretty late into Gen 5's life cycle (And by that I mean X and Y was literally announced and coming out shortly afterwards) so I've kinda just been playing the newer games instead.

This is difficult for me because I've never really found any of the games to be especially difficult. Even on hard mode, I completed a solo nuzlocke of White 2 using only my oshawott > dewott > samurott, but the games aren't geared towards being especially difficult because they're for all ages. That being said, the Ultra Necrozma fight in USUM was brutal, even if I beat it the first time because I knew it was coming.

From a more humorous perspective, gen 7 was the hardest to play because it was so damn boring from a gameplay perspective imo. I still haven't finished UM (I finished US, but UM... I can't stand to sit through Alola again as much as I love the concept of the region).

None of them are that difficult, but I don't build my teams very thoughtfully. Playing through Heart Gold 4 members of my team were weak to Water/Ice, causing me trouble at certain points like Misty. If I planned and thought ahead I would probably rip straight everything.

Directly answering the question I would guess Emerald was the most challenging, but on my last play through I had no trouble.

The Johto games for sure, namely HGSS since those are different by nature than the rest of the main series games, therefore not being "used to it". Aside from that, as lame as an excuse as that is, I would probably go for Emerald funnily enough. Found it hard to get by without looking at guides and walkthroughs, perhaps my youth was at fault for that. Now it is the game I still tend to play the most out of all series games but I guess life is funny like that.

HG/SS is BRUTAL. Between the amount of important TMs locked behind Voltorb Flip, the fact that most of the evolution stones are locked behind the Bug Catching Contest, and of course, the difficulty of grinding in the latter portion of the game, I find HG/SS to be completely unplayable without an Action Replay.

Pokemon Platinum proved a great challenge to my 8-year-old self, so did Diamond and Pearl but lesser.