I decided to look at the ones that might raise a few eyebrows in the thread only. Some of the B / C ranks can simply be explained in a line or two, & therefore I just decided to do the S & A ranks for now.
Rhydon, to me, is pretty strongly ahead of any other competitor, but not to the point where I would consider it to be nearly mandatory, something that Gligar!Gyarados could claim. Defensively, it is near enough the best wall in the metagame, with Salamence / Dragonite sets being able to take on some otherwise shockingly powerful threats like Magmar!Gallade, Zacian-Crowned, Gyarados!Farfetch'd-Galar, and Dragonite variants are even capable of taking on the infamous Munchlax!Arcanine sets. Gallade!Rhydon itself is an incredibly potent wallbreaker, even being able to beat common physical walls like Rhydon!Salamence and Wailmer!Milotic. Gallade inheritors also got an upgrade this generation, now being immune to Intimidate, making this Rhydon even harder to beat. Scolipede!Rhydon is also quite alike to Gallade, except its main difference is that it's an extremely powerful cleaner. The only problem with these sets tends to be that bulky waters, grasses, and Rhydon itself get in the way.
Zacian-C is still a very good mon, but in Cross Evolution, the likes of Rhydon, Scyther, Wailmer, and various other offensive / defensive threats actually kick Zacian-C down a notch to the point where it isn't really the single best mon in existence. Still a pretty strong tank regardless, but not on the level of Rhydon imo.
Electabuzz is honestly kinda broken, but that's because it abuses two unhealthy donors: Butterfree and Gallade, both of which catapult Electabuzz from being a decently powerful cross evolution to being one of the deadliest in the game, forcing respect to be made with its presence on the field. Electabuzz!Butterfree is exceptionally strong with its high Speed and Special Attack in addition to its access to Quiver Dance, giving Electabuzz the ability to sweep with ease. On top of this, it also grants a Flying typing and two great abilities in Compound Eyes and Tinted Lens, making it either more reliable with 3 attacks or giving it the ability to just straight up make people angry with Sleep Powder. Gallade variants also are extremely obnoxious to deal with, only really being beaten by certain Fairy or Psychic-types due to its solid speed tier (135) and excellent coverage. Even if some Psychic-types are decent pivots, they can also be beaten by Electabuzz!Gallades carrying Knock Off.
Scyther is not quite on the level of Rhydon but its sets are still both amazing. Scyther!Excadrill is effectively this metagame's Duskmane, being capable of checking a huge amount of otherwise deadly threats into a single slot, whilst also providing reliable Stealth Rock setting and being an excellent defensive pivot. But if Steel-types aren't your game, then Scyther!Urshifu-R might feel better for you. Scyther!Urshifu-R has an exceptionally anti-meta typing, being able to take on the majority of Ground types, even despite its Flying weakness, and alongside this, has a rather difficult to wall STAB in Surging Strikes, and when combined with Knock Off / U-turn, it can make answering Scyther!Urshifu-R a bit of a chore. Likewise to the rest of S rank, this demands attention in the builder.
A Ranks:
Calyrex-Shadow is an exceptionally strong mon but I find it to just be slightly off the effectiveness of Zacian-Crowned and Electabuzz. Both of those mons are extremely threatening offensively, and like with Calyrex, demand a lot of attention in the builder, but Calyrex-Shadow's perpetual weakness to Knock Off as well as the addition of pivots like Wailmer, certain Milotic donors and Porygon2 kick it down a notch.
Whenever I think of a wall in Cross Evolution, Wailmer is pretty much the immediate answer I would give. It has gargantuan bulk, good utility in either Knock Off or Flip Turn and overall is simply the best answer to some of the crazy offensive mons in the metagame like Kyogre and Calyrex-Shadow. Not a whole lot to say about Wailmer, you'd just have to believe me on this one.
Porygon2 is an exceptional pivot / bulky setup user. All of its four sets are great at just being annoying tanks that are either capable of bouncing away hazards from even the best of setters (excluding Scyther), capable of being one of the few viable users of Clangorous Soul + Recover, making it an exceptionally powerful snowballing tool, or just being really frustrating with CM. Excellent blanket check to some of the better special attackers in the metagame too, like Calyrex-Shadow & most Nidoking / Clefable donors.
Vullaby!Gyarados is actually exceptionally annoying. For a DD user, it is really slow, but between its excellent longevity, access to Knock Off, Brave Bird & Roost naturally, and solid overall metagame coverage, it ends up being a rather difficult Gyarados donor to stop. Even with Quagsire in the meta, its Brave Birds often do too much damage, and in regards to other checks, Weakness Policy variants will often get the boot versus it due to being unable to OHKO it. Solid mon.
Kubfu's niche over Farfetch'd-Galar is a bit of a weird one, but put simply: I believe Kubfu to be the better sweeper, whilst Farfetch'd-Galar is the better wallbreaker. Kubfu's high Speed for a Fighting type, and ESPECIALLY a Gyarados user make it much easier for it to pull sweeps off, especially without Screens support, and its access to Acrobatics & Drain Punch allow it to stay alive for longer compared to other donors which may either have issues with dropping their item or may not have access to draining moves.