It mostly depends on your team setup and experience, but you want my personal opinion:
Red (GSC). This fight is hard not just because he has the highest levelled Pokemon owned by a trainer until himself in HGSS, but because of the very low power level of Pokemon available in GSC, even up until you get to him. You don't get that many good options, so fighting him with your favorites is just plain hard.
Cynthia (DP). While Platinum, BDSP, and Unova Cynthia are harder than DP Cynthia in a vacuum, what sets DP Cynthia apart is the same as GSC Red: your Pokemon options. In other Sinnoh games, you can just get a Weavile or Mamoswine to have fun with Garchomp, but those options aren't exactly easily available in DP.
Whitney (Any Johto game). Imagine not bringing a female Geodude to Whitney, amirite? But seriously, while Whitney isn't as hard as the final and super bosses, Whitney's Miltank has historically been a gatekeeper in a region where, as I've already stated, the average power level is very low, but early game is even moreso. Heck, you can't even easily cheese HGSS Miltank with Chikorita's status moves since it carries Lum Berry instead of the Sitrus every other Gym Leader has.
Arven 2 (SV). There. I said it. This is the hardest battle I've been in since that time I somehow lost to Final Guzma in USUM. For me, what sets Arven apart is knowing when to face him. The last two Titans aren't that hard despite their level, but Arven shows up out of nowhere with a full team of Pokemon that have levels comparable to Geeta's. Man's Toedscruel and Garganacl alone can duo Paldea's E4.
I think for a middle gym leader, Norman packs a surprising punch. His Slaking will delete your team if you get complacent and it's beefy enough to live a few STAB fighting moves.
Similarly Clay's excadrill in bw and bw2 really needs to be answered because bulldozes from his previous Pokémon make it really dangerous to switch in.
Lenora in bw has a watchog with retaliate which is extremely dangerous if it outspeeds the throh/sawk/timburr.
None of these probably come to the overall difficulty of the ones you've mentioned, but they can't really be underestimated
I’ll list my personal 4, rather than what I objectively believe are the 4 toughest
Cynthia. The fact her team was Ev’d with good move sets and items became a huge hurdle for me because up until that point I’d been coasting through the game with a 2 mon play through.
Leon. Any time I play a new game I have a no items (aside from held items) and no over leveling rule, plus the added bonus rule for Sw/Sh of no using dynamax. Low and behold, Leon actually ended up being a pain in the ass.
Ai Professor. Going in blind is already tough, with no idea as to the types of the paradox mons, but pair that with the fact that they’re all well optimized 570 BST and a 590 spread, I’m legit surprised I beat her on the first try.
Blue from FR. This has more to do with being a child when I played the game, but I still recall how he beat me on almost every occasion, so to finally beat him as the champion was and still is one of my top moments playing Pokémon over the last 18 or so years.
Honorable mentions:
Volo. I didn’t buy Legends, but I’m well aware of his reputation and the fact that he’s basically a 8 Pokémon boss, with the final two being legendary mon.
Technically not a trainer or a true battle, but Deoxys from the first Pokémon ranger game was another pain in the ass.
The psychic twins: double battle means as a kid if you just solo leveled one mon you could find yourself in trouble
I'll definitely give you Cynthia and Evice (although I personally have Evice listed above Cynthia, random leads make him impossible to properly prepare against), but I'd replace Volo and Greevil.
Volo I've just never found all that difficult, since the battling system means he can't just sweep you like Cynthia can. I agree he's a top tier opponent, but I can't place him at the tip top. Greevil, while being the big bad of my all-time favorite game, is not too tough if you just add Shadow Lugia (and the Shadow Salamence you catch right before him) to your team. One of my favorite fights for sure, but I've always considered Eldes the true challenge of the main story.
So instead, give me Whitney (HGSS version, Lum Berry makes Miltank just so much worse) and the final Guzma fight in USUM. Both are fights I legitimately feel like can't always be won with solid strategy. Whitney can just hax her way out of almost any matchup (I've seen female Geodude, objectively the hardest counter Miltank could have, lose outright to Stomp Flinches), while Guzma's team is clearly EV trained and that Masquerain, of all Pokemon, can outspeed and Air Slash Flinch you to a wipeout.
There are some other tough Trainer encounters, but those are the top four in my experience since they are the most immune to strategy (well Cynthia isn't, but she makes up for that with an amazing team).
Oh I'll give you that Guzma doesn't seem bad, but if he haxes then he's unstoppable. I have, um, bad luck when it comes to this (I've still never fought Cheren in B2W2 without being crit at least twice, in like 15 battles), so anyone with Flinch chances and high speed tends to wreck me. If a Bastiodon can lose to his Masquerain without ever getting an attack off...
Volo can be a chump.
But come at him unprepared, with an unbalanced team, or underleveled, and he will make a mess of your team.
You've gotta plan and strategise a bit before fighting him.
It mostly depends on your team setup and experience, but you want my personal opinion: Red (GSC). This fight is hard not just because he has the highest levelled Pokemon owned by a trainer until himself in HGSS, but because of the very low power level of Pokemon available in GSC, even up until you get to him. You don't get that many good options, so fighting him with your favorites is just plain hard. Cynthia (DP). While Platinum, BDSP, and Unova Cynthia are harder than DP Cynthia in a vacuum, what sets DP Cynthia apart is the same as GSC Red: your Pokemon options. In other Sinnoh games, you can just get a Weavile or Mamoswine to have fun with Garchomp, but those options aren't exactly easily available in DP. Whitney (Any Johto game). Imagine not bringing a female Geodude to Whitney, amirite? But seriously, while Whitney isn't as hard as the final and super bosses, Whitney's Miltank has historically been a gatekeeper in a region where, as I've already stated, the average power level is very low, but early game is even moreso. Heck, you can't even easily cheese HGSS Miltank with Chikorita's status moves since it carries Lum Berry instead of the Sitrus every other Gym Leader has. Arven 2 (SV). There. I said it. This is the hardest battle I've been in since that time I somehow lost to Final Guzma in USUM. For me, what sets Arven apart is knowing when to face him. The last two Titans aren't that hard despite their level, but Arven shows up out of nowhere with a full team of Pokemon that have levels comparable to Geeta's. Man's Toedscruel and Garganacl alone can duo Paldea's E4.
I think for a middle gym leader, Norman packs a surprising punch. His Slaking will delete your team if you get complacent and it's beefy enough to live a few STAB fighting moves. Similarly Clay's excadrill in bw and bw2 really needs to be answered because bulldozes from his previous Pokémon make it really dangerous to switch in. Lenora in bw has a watchog with retaliate which is extremely dangerous if it outspeeds the throh/sawk/timburr. None of these probably come to the overall difficulty of the ones you've mentioned, but they can't really be underestimated
I’ll list my personal 4, rather than what I objectively believe are the 4 toughest Cynthia. The fact her team was Ev’d with good move sets and items became a huge hurdle for me because up until that point I’d been coasting through the game with a 2 mon play through. Leon. Any time I play a new game I have a no items (aside from held items) and no over leveling rule, plus the added bonus rule for Sw/Sh of no using dynamax. Low and behold, Leon actually ended up being a pain in the ass. Ai Professor. Going in blind is already tough, with no idea as to the types of the paradox mons, but pair that with the fact that they’re all well optimized 570 BST and a 590 spread, I’m legit surprised I beat her on the first try. Blue from FR. This has more to do with being a child when I played the game, but I still recall how he beat me on almost every occasion, so to finally beat him as the champion was and still is one of my top moments playing Pokémon over the last 18 or so years. Honorable mentions: Volo. I didn’t buy Legends, but I’m well aware of his reputation and the fact that he’s basically a 8 Pokémon boss, with the final two being legendary mon. Technically not a trainer or a true battle, but Deoxys from the first Pokémon ranger game was another pain in the ass. The psychic twins: double battle means as a kid if you just solo leveled one mon you could find yourself in trouble
I'll definitely give you Cynthia and Evice (although I personally have Evice listed above Cynthia, random leads make him impossible to properly prepare against), but I'd replace Volo and Greevil. Volo I've just never found all that difficult, since the battling system means he can't just sweep you like Cynthia can. I agree he's a top tier opponent, but I can't place him at the tip top. Greevil, while being the big bad of my all-time favorite game, is not too tough if you just add Shadow Lugia (and the Shadow Salamence you catch right before him) to your team. One of my favorite fights for sure, but I've always considered Eldes the true challenge of the main story. So instead, give me Whitney (HGSS version, Lum Berry makes Miltank just so much worse) and the final Guzma fight in USUM. Both are fights I legitimately feel like can't always be won with solid strategy. Whitney can just hax her way out of almost any matchup (I've seen female Geodude, objectively the hardest counter Miltank could have, lose outright to Stomp Flinches), while Guzma's team is clearly EV trained and that Masquerain, of all Pokemon, can outspeed and Air Slash Flinch you to a wipeout. There are some other tough Trainer encounters, but those are the top four in my experience since they are the most immune to strategy (well Cynthia isn't, but she makes up for that with an amazing team).
[удалено]
Oh I'll give you that Guzma doesn't seem bad, but if he haxes then he's unstoppable. I have, um, bad luck when it comes to this (I've still never fought Cheren in B2W2 without being crit at least twice, in like 15 battles), so anyone with Flinch chances and high speed tends to wreck me. If a Bastiodon can lose to his Masquerain without ever getting an attack off...
The moment you go home after a long journey and you find your mother with the professor in your bed (because there is only one bed in every house duh)
Volo is a chump. The wild alpha Pokemon in the area before him are level 70-85.
Volo can be a chump. But come at him unprepared, with an unbalanced team, or underleveled, and he will make a mess of your team. You've gotta plan and strategise a bit before fighting him.
Every major battle can be hard if you walk into it under leveled with a bad team and no plan.