gonna post some of my takes on the tier with explanations for anything that I have a lot of experience using and playing against
Ban:

Obviously this is a super early thing to say, but when exploring defensive options for this thing they're extremely limited and nearly impossible when factoring Tera compared to the ease of use this thing has. Tera Ice sets can muscle past raw physical walls like Bellibolt while Tera Grass and Electric obviously break through bulky Water-types like Vaporeon and Slowbro. Cloyster's bulk, especially paired with Screens makes it far too easy to set up for how difficult it is to manage.

Haxorus, much like Dragon Dance users in tiers above it, is a prime example of what can break Tera. Lose to Sableye and Slowbro? Tera Dark. Lose to Fairy- and Ice-types? Tera Steel. These are the two best examples but Haxorus has a very malleable movepool and bulk. Haxorus can also use Swords Dance with Trailblaze probably more effectively than Dragon Dance but that's up for debate.
S tier:

Keeps the broken Fighting-types like Flamigo and Tauros in check and stays healthy with Slack Off and Regenerator. Calm Mind sets can take over games quite easily, bolstered further by Terastalizing. Yawn and Thunder Wave can also be spammed with no commitment in a tier with very little answers to status. Future Sight pairs incredibly well with Dark-types like Weavile and Krookadile for essentially guaranteed KO’s when set up properly.
A tier:

Fantastic offensive presence even when defensively oriented, able to check a lot of the tier and reliably spread burns. Offensive sets are also quite good in their own right, with either Choice Band or Cud Chew strats. Intimidate and Fire-Fighting typing lets Tauros check big threats like Weavile, Slush Rush sweepers, and Heracross.

Bellibolt can effectively 1v1 most of the tier with Soak, Charge Beam, and Parabolic Charge. It's bulk lets it even stifle out setup sweepers like Scarf Heracross and Cloyster. Soak while Charged often forces an immediate switch, making hazard stacking insane when paired with Bellibolt.

Particularly Choice Scarf sets are amazing offensively in a tier lacking good Steel-types while Trick and Trace give Gardevoir amazing utility.

I think the Shift Gear Tera-Normal set is quite overrated, but Choice Specs is nearly uncheckable and Shift Gear sets are still good, just not the auto-win button some people make it out to be.

Insane Snow core, Cetitan is very difficult to check, especially with Tera. I've used Tera-Dark to wall Sableye but Tera Ice can make for even stronger +6 Ice Shards and Fairy, Fire, and Ghost can make for great defensive Teras to enable a safer Belly Drum.

(Heat and Mow)

Choice Band with Beat Up completely dunks on all the dedicated leads and besides Tauros and Arcanine can 2HKO almost the whole tier and even has Ice Shard to pick off weakened Scarf users and setup sweepers.

The SSNU IronPress set that I made way back when is still super dumb. Basically the only good Steel-type with SR and good coverage too.
Other stuff I haven't used but seems A rank worthy when playing against or just thinking about it:
Stuff I think is B rank worthy or below.
Sorry for the watered down second half of this post I got tired :0