I don’t know how controversial this is, but I question Zamazenta’s placement as top 1. The tier isn’t exactly scrapped for good fighting type options or revenge killers, nor is preparing for Zama anywhere near difficult. Zama’s coverage is very middling. A BodyPress Zama Crunch isn’t even favored to 2HKO minimum invested Gholdengo, and stone edge isn’t a 2HKO on defensive zapdos. This alongside it having huge 4MSS makes an unsupported Zama very easy to pivot around - not exactly the hallmark of a top 1.
In particular I think the BodyPress set is overrated. Its coverage issues are exacerbated even more when its not investing in its attack, and if you can’t switch into a boosted BP, then your team probably has problems with other cleaners like Iron Valiant. It just gets walled too easily by very common mons, and is destroyed by status hard (though tera does alleviate this). Dragonite and Kingambit are far more splashable than Zamazenta in the cleaning department, and while yes its a great revenge killer, there are a lot of those.
Zamazenta is the single most splashable pokemon in the tier and it's not really close. You say "tier isn’t exactly scrapped for good fighting type options or revenge killers" but Zamazenta is bar none the best fighting type in the tier, and is a significant source of role compression, and nothing else quite does what it does. It provides a soft check to a huge range of threats all in one slot because of its near matchless speed tier (only surpassed by Dragapult and Deoxys-S naturally) and great natural bulk. It's speed control, and it does this very well. The tier is riddled with excellent dark types which it handles well, and there's no shortage of potent physical attackers which Zama provides a check to. I mean off the top of my head, this mon soft checks
Kingambit, Great Tusk, Kyurem, SD Gliscor, Ogerpon-Wellspring, Ceruledge (using Roar to phase, but ID also generally matches SD barring crits), Darkrai, Samurott-Hisui, Ogerpon-Teal, Rillaboom, Ursaluna.
That's with standard IDBP sets btw. All-out-Attacking sets can even emergency check stuff with fast invested CCs or assorted coverage. Of course it doesn't do this all on its own, it'll get overwhelmed if relied on solely. But it can take the burden off of other mons on the team if necessary in a game.
As far as issues with IDBP, yeah it's something that isn't hard to prep for in a vacuum, until you realize most things used to check it are relied on to handle other threats as well that are found on Zama's team, so they have to be very careful not to get too worn down or worse, KOd, before Zama is down. Because of this, IDBP still commands a large amount of respect as a wincon and forces players to play around its presence right from the start. And with different Teras and move changes, it can just make its way past a lot of supposed counterplay anyways. Rest sets are a good example of this.
Dragonite and Kingambit ARE technically more dangerous, but that has more (mostly actually) to do with Tera Blast letting them cut past counterplay. Zama as a whole does so much in one slot, more than basically anything I'd say, and is an extremely good high value mon. Like I said, there really isn't another mon that does what it does.
What is the argument against having Great Tusk rated S? As someone who doesn't have the most experience with gen 9 OU, I'm genuinely asking.
From my perspective, it is the MOST viable Pokemon in the tier, as it is easily the best hazard remover and arguably the best SR setter as well. It also fills essential defensive and offensive roles for every team: physical wall, electric immunity, and offensively checks or forces teras from other top threats such as Ghold, Kinggambit, Raging Bolt, Dragonite, etc.
It has phenomenal STAB moves, excellent all around coverage, and great utility options if you want (knock off, taunt, and roar are all uncommon on Tusk mainly because its main moveset is so dang good). It has multiple viable sets (offensive utility, booster energy, scarf, bulk up, defensive helmet) and sees widespread play on basically every archetype except stall.
My sense is that people take for granted how important Tusk is in OU, particularly in regards to setting and removing hazards. The usage stats reflect this too as Tusk clears the rest of the meta by a wide margin. Very curious to hear the argument for keeping it in S- as opposed to S.
I'm inclined to agree. Zama is very good but I don't think it puts more strain on teambuilding than other top threats like Wogerpon or Gholdengo, particularly in a metagame where tera ghost is such an easy bring.
Also gonna touch on this quickly but Tusk is fine in S- and not S. It's absolutely very splashable and useful for a wide range of structures, but it isn't something a team outright NEEDS and plenty of teams go without it. It does have competition for its teamslot at times, and its defensive typing is poor and quite exploitable. It suffers quite a bit from 4MSS at times, and can really struggle to fit all you'd want. No Ice Spinner is food for Gliscor (especially Toxic) as well as Landorus-T and Dragonite. No Knock Off causes issues with making progress into bulkier teams whose Tusk check is very sturdy with Boots (Zap/Molt). While it is more common now, some don't pack Close Combat and lacking it means Ogerpon-W can quite nicely switch in (also lacking CC means you don't instantly threaten Kyurem even if you're speed boosted).