Hi! We currently have a VR, though it is quite outdated since several of the Pokemon there have had viablilities based on previous metas (sun abusers like Leafeon and Scovillain are much more niche, Sneasel being underexplored). ZU council intends on working on a VR soon for ZUPL, however, so stay tuned for updates.
Decidueye-Hisui's high usage is indeed supported by its excellent utility movepool and helpful defensive stats. The variety in its support toolkit includes Knock Off, Defog, U-turn, Roost, and even Night Shade and Taunt to back up its defenses. While its Speed is low for an offensive Pokemon, the high Attack, typing, and versatility with moves like Triple Arrows, Sucker Punch, Swords Dance, the aformentioned Knock Off + U-turn, and even moves like Grassy Glide for Grassy Terrain teams makes it a great fit for many teams, offensive and defensive alike. As it stands, Decidueye-H continues to be a prominent force in the current metagame despite gaining two new checks in Whimsicott and Grafaiai.
Charizard and Typhlosion now sit in a worse spot due to Drought's ban. Charizard has ran specially offensive and Belly Drum sets to decent success, though it fails to live up to the success it had on sun teams. Nevertheless, it still remains a decent attacker here. Typhlosion has been fairly strong thanks to its solid Speed and not being as reliant on sun to function, though unlike Charizard, it's easier to fit on teams without a 4x weakness to Stealth Rock and usually being consistent with Eruptions thanks to Boots. Kingdra has also banned as of our recent suspect. And yes, Alcremie and Snorlax continue to become elite walls and wincons in the tier; Alcremie in particular remains a focal point thanks to its typing, bulk, Recover, and Aroma Veil being very consistent and easy to set up and close out games.
Samurott is in a weird spot where it's not bad per se, it just faces a ton of competition with other offensive Water-types like Bruxish, Clawitzer, Qwilfish, Veluza, and even Poliwrath for a slot on teams. Its Speed is in a crowded spot where standing up to the power crept meta here requires even more standout qualities, and while I think Samurott is perfectly usable to dismantle slower builds with SD or mixed sets, the aformentioned Water-types above (esp. Bruxish) just tend to be easier to get consistent milage out of. Be it power, Speed, a secondary typing...but again, it's not bad, it's just hard for it to distinguish out of the other Water-types.
Meganium sits alongside memes like Regigigas and Mightyena for how outclassed it is by...literally every other Grass-type and defensive Pokemon. You're not wrong about the worst starter here, as soon as it dropped here it never saw use.
I think the comparison is going a bit on apples and oranges. In terms of viability? They fulfill different roles here, as you can't really compare Charizard's offensive prowess with say, Decidueye-H's defensive utility. But based on how well they fulfill their roles and how they fare in the meta, I'd switch around Zard and Typhlosion's placements and you'd be good to go.
Hope this helps!
unranked? Great Atk and Speed tier, good ability if you are able to predict Volt Switch from the new Rotom-Mow for example and great coverage options in my opinion (being even possible to use it as an special attakcer)