I think the first thing we should do is universally agree on the big battles in the game, the ones that way the most: again, basically talking Platinum specifically.
I'll describe how I value them, and others can give further input to how much weight we should give to each major battle. I'll go through and potentially list your best checks if applicable for each with my current knowledge. Gonna be using a scale of:
Irrelevant (Very exploitable, common counters run roughshod over them)
Pretty Easy (Has decent mons but they probably won't sweep you)
Important (Notable battle with no real holes in the team that you can easily exploit)
Threatening (Potential for a total party kill, just not on the level of Champion)
Champion (Hardest fights in the game by far)
Barry
Really only notable from Canalave onwards, and even then...I honestly think we could skip testing Barry as a whole if only for convenience. The Staravia and second stage starter are the only things of note in the Hearthome and Pastoria fights, making him roughly on par with a midgame Ace Trainer (Near Jubilife fight is also pretty pointless as most members won't be around for it).
Heck, even the Canalave stuff really isn't even THAT scary.
Aqua Jet Floatzel? Easily resisted Roserade? No Close Combat on Staraptor? Explotable Heracross? FIRE SPIN RAPIDASH? Meanwhile his potential Torterra has...Razor Leaf, Mega Drain, Leech Seed, and Bite. No Earthquake.
The final battle at the League have competent movesets, but I also think we could exclude it for the sake of consistency. Not putting another screenshot because I want to keep this post as short as is. Currently I'm feeling...
Pretty Easy.
Roark
Honestly, while not one the hardest gyms, I think this one can still be a minor challenge thanks to Cranidos literally 3HKOing anything with Headbutt. The Rock / Grounds are sorta fodder though. Typical early Rock leader in a good chunk of early stuff won't be effective. You have Machop and the starters, Geodude needs some grinding and even then likely only helps against Cranidos with Rock Throw, Budew I guess but raising a Budew is awful thanks to a friendship evo so...I'm gonna go with
Pretty Easy (if only for the fact most will use a Sinnoh starter / easy workarounds).
Mars
This one is weird because the first fight is scary, the second fight is a minor speed bump, and the Double Battle is kinda pointless. I'll label the first battle
Important, the other two border on
Irrelevant because the average team can easily stand up to Bronzor and Golbat + ace by then. Purugly is a pain in the first fight, but you have Machop, Staravia and heck, maybe even Geodude and I don't think it's as egregious as...
Jupiter
Double battle is again
Irrelevant, but that first fight could honestly go under
Threatening. I almost want to say Champion, but...you have Staravia Intimidate, you most likely have 3 members at this point, it's more or less your team versus a Skuntank. Hard to stomach hits from, but it can also derp with status moves. You'll win by a hair, sure, but it's not like Mismagius (every single move is threatening). And yes I know this comes after Gardenia, putting it here because her and Mars 1 are pretty infamous early game roadblocks.
Gardenia
Deceptively difficult. You have way more counters in comparison to previous bosses (Vespiquen, Ponyta Beautifly, Monferno, Staravia) but Turtwig's Reflect and Cherrim's Leech Seed make it hard for said counters to make progress, so you mostly get by in this fight with resists or neutral matchups...which can still get chunked by Grass Knot (I think Grotle is 3HKOed and another check in Gyarados is pretty heavy). In spite of all the counters I'm honestly gonna say
Important because so many physical options aren't exactly easy answers if Reflect or Stun Spore gets going.
Fantina
Champion. Straight up, if almost only for that Mismagius. Your four checks are Floatzel with Crunch, Gyarados with Bite, Shinx line (lul) or bizarrely Toxic + Heal Order Vespiquen (guys Captivate is really reaching in my opinion, plus anything can kinda do it). Duskull screws over so many physical attackers with Will-O-Wisp and Rawst Berries are VERY out of the way under Cycling Road. Mismagius is WAY worse than Skuntank, the coverage easily hits a good 60% of your options super-effectively, and the ones that don't die instantly can't kill it back at all.
Maylene
This one's odd. You're unlikely to sweep her thanks to Lucario killing a few counters that beat her other mons, but RELATIVE to other Gym Leaders Maylene really isn't all that scary. Part of me wants to say
Important but I think she's easier to roundhouse than Gardenia despite her packing Rock Tomb on two mons (mons off the top of my head who can make progress: Starly in general, Driftblim, Kadabra, Gradevoir, Machoke / Machamp (Lucario), ground types like Hippopotas / Gliscor / Gabite with Earthquake (Lucario)). Let's say
Pretty Easy.
Wake
Even some of the best options for a Platinum run have trouble making progress against Waterfall Gyarados and that quick Floatzel, not to mention his high levels. Quagsire is a freebie but it's rare you'll find something that's great here that sports a lot of advantages elsewhere (pretty much any Electric like Shinx / Jolteon, Grasses struggle to win the damage war on Gyarados and Floatzel has Ice Fang). I almost wanna say
Champion but the answers here are fairly straightforward as opposed to Mismagius where a few things rely on non-STAB Bite / Crunch. So we'll go
Threatening (you can get resists here as opposed to Mismagius which is practically unwallable).
Cyrus
First two fights are
Pretty Easy (I honestly think they could be argued for
Irrelevant if Sneasel / Murkrow line didn't hit like atomic bombs) but that third fight is another
Champion. Virtually impossible to sweep the Spear Pillar / Distortion World fight, though fast Electrics can really help there.
Byron
Go to
Irrelevant and think about what you did. His ace's best option is friggin Metal Burst, and a typical Specs Surf user can overwhelm Magneton and Steelix. Literally every starter laughs at him, as does Staraptor, Golem, Machamp, Gliscor, Specs Surf users...you get the point. Most Waters should take one Thunderbolt from Magneton unless they suck.
Saturn
This guy is a Galactic Admin? After the first two? Both fights are
Irrelevant. Not even the Toxicroak is that scary.
Candice
Yeah, she's
Important. Threatening moves? Check. Evasion spam being annoying on Froslass? Check. Bulky mons in Piloswine and (if you don't hit its weaknesses) Abomasnow? Yeah. There's a few counters here (Medicham, Machamp, Empoleon, Infernape, heck even Staraptor puts in work with Close Combat) but a good few will fold to Froslass being faster and chip damage throughout the fight, and she has an answer for you if you fail to OHKO your counter to her. You could argue
Pretty Easy, but not sure.
Volkner
This guy is kinda hilarious. His coverage is more scary than his actual STAB. You have multiple ways of getting past him: Ground types with Earthquake, Calm Mind users like Alakazam or Gardevoir, no less than three Water / Ground types in the game. Odds are, you should have an answer to him. Hilariously, Torterra folds to him without Rock Polish. I almost wanna say
Irrelevant but we'll go
Pretty Easy.
Aaron
His mons in Platinum are honestly pretty threatening for a Bug type user, but he also has multiple quad-weaknesses. I almost wanted to say
Important, but we'll go
Pretty Easy, he's not that hard to wall outside of Heracross and this game also has Staraptor in it, lol.
Bertha
Yeah, she's
Pretty Easy. Again, so many quad-weaknesses and on top of that, most of her team is pretty slow.
Flint
Almost wanted to say
Threatening, but we'll go with
Important since that hasn't gotten much love. Only two members have Sunny Day: Houndoom and Rapidash, so should kill them fast if possible. You're unlikely to muscle through his entire team with one mon, but despite his coverage your Ground types and Water types should still do good here, though his coverage is way scarier than anything Aaron or Bertha threw at you.
Lucian
Threatening for sure. Not much in this game walls Psychic + Fighting coverage, Reflect + Light Screen messes up sweep attempts, and he's by far the scariest of the four with both setup and offensive pressure.
Cynthia
Champion Cynthia is
Champion difficulty, this is an obvious one. Traditional counters get screwed by coverage, but unlike with the other Champion fights, I don't care too much if something loses here. Sweepers here amusingly include Dragon Dance Gyarados, Swords Dance / Rock Polish Rhyperior, and Simple Bibarel. But yeah, she's hard.
What do you all think? I rated quite a few
Pretty Easy, but meh.