Tera: Fighting, Dark
Like with most Ghost types, you get a Dark resist and with Tera Blast Fighting you get really good STAB combo. Though you can use Dark as well since Banette has one Fighting option and a bunch of good Dark moves to abuse.
Tera: Water, Grass
EBIN MEME DAMAGE aside, I think Kyogre would benefit a lot with Grass or Electric. While its Super STAB Water attacks sound Ebin, Kyogre doesn't need damage from Water Spout or Origin Pulse so much as it would need cover. Grass benefits Kyogre as a means to hit its Ubers nemesis, Gastrodon super effectively with Grass Tera Blast. Grass of course covers its weaknesses as well.
Tera: Fire, Water, Dragon
Well guys, Primal Groudon finally returns So Groudon has a new partner/Rival in Koraidon. They pair up very well actually as Groudon is more defensive and Koraidon is by far more offensive and its typing matches well for a Sun team. Unlike other pure Ground types, Groudon has Drought, making Steel or Grass counter intuitive. Fire is a pretty good option for Groudon seeing how great its Primal form was. Drought halving its Water weakness and letting it use STAB Fire moves under the Sun. It also resists Ice and Fairy for Koraidon when paired with it. Water and Dragon provide a resistance to Water without being weak to Fire so that it can easily switch into Kyogre.
Tera: Steel, Ground, Flying, Water, Dragon
Ok, so this Gen Rayquaza lost one of its GOAT tools in V-Create due to transfer moves being killed. We got rid of Scald, but at what cost? So that kind of ruins the idea of Fire Rayquaza if you don't want to use Overheat. Steel and Water are the Ice protection for Rayquaza and have their own benefits. Ground protects it against Rock, but also lets it have STAB Earthquake for Steel types and for when Necrozma returns for DLC. Dragon and Flying can actually be used with Draco Meteor and Dragon Ascent respectfully if you want too.
Tera: Bug
To be frank, I have no clue what to do with Kricketune since it's just awful. It has no movepool and can't be saved by Tera Blast either.
Tera: See Overqwil, but with Fire over Water, and add STABs
Skuntank like Overqwil doesn't really get much from Terastallizing since it already only has 1 weakness and Bug/Grass/Flying otherwise provide a bunch of weaknesses. It can also use it more offensively with Fire and its STABs already.
Tera: Steel, Poison, Fairy, Dark
Uxie has a great position due to being a Floating Defensive Psychic. It can take advantage of Levitate by becoming Ground Weak types like Steel or Poison, and it can additionally use Fairy or Dark to help with its regular weaknesses too. It's pretty simple since it is a more defensive Pokemon.
Tera: Very similar to Uxie but with Electric
So Mesprit is a well rounded Pokemon that doesn't specialize in anything, so it's a bit more offensive than Uxie. As such it would use all its Teras and also Electric for Thunderbolt STAB. Sadly lacking a lot of move despite being the well rounded one of the Lake Trio.
Tera: Same as Mesprit but with Fire
Azelf's only change to the other Lake Trio are its movepool and stats, which it actually gets a bunch of useful tools naturally.
Tera: Grass, Flying
I did Heatran in another list even further back describing why and how it would use Grass and Flying. To summarize, Heatran really benefits from Grass due to its match up with Rain teams and also Ground spam. Heatran is a great example of Terastallizing defensively and how to use it. As it's not so much about making a Pokemon a super power like Dynamax, but more so giving yourself a way to have better odds against your match-ups. You can choose to Terastallize a Pokemon that would be weak to a certain match-up normally, but can turn out good with a new typing. And then when it isn't facing those match-ups, it can remain its original typing. That is to say Heatran Terastallizes when it needs to and stays Fire/Steel when it needs to.
Also Flying is a great option as well, but it is weak to Stealth Rock and Grass pairs better with Flash Fire.
Tera: Same a Uxie
It's literally just Uxie but bulkier for less speed and tools like U-turn.
Tera: Literally anything technically
So Arceus is a unique case as it can literally be any type it wants already, which you think would make Terastallizing useless, but remember what I said about Heatran. You can effectively have 2 different Arceus forms on your team and you choose which one to use each match (unless it doesn't override Plates or something. Idk how it works with Arceus).
Tera: See all the generic Psychic types normally
Gothitelle outside of Shadow Tag is really nothing special that separates it from other Psychics. Not even something like Levitate.
Tera: Water, Electric, Fighting
Being an Adaptability user, Dragalge definitely wants to use Terastallizing to its full potential and use 3 STABs that are all boosted by x2. Honestly don't know what happens when you do Adaptability+Super STAB. I was told it would be x2.66~, but idk based on how Adaptability and Tera are coded so uniquely.
Tera: Flying, Water, Electric
Klefki is nothing special. It has the same type as Magearna so it doesn't benefit much from Terastallizing. It can use Flying or Water to take a would-be super effective hit in a pinch, and it can do that with Electric and Magnet Rise too.
Tera: Fighting, Fairy, Dark
Hoopa in the lower tiers has great potential using this mechanic. Being a Ghost/Psychic type, it is extremely weak to Ghost and Dark, and its low physical bulk. However, it has great special bulk to make up for this and with Teras it can be much more of a threat. While Pursuit is gone, Hoopa still appreciates losing its Dark and Ghost weaknesses with most other types. Fighting is for Focus Blast STAB. Fairy only has Tera Blast for STAB, but it does provide a great defensive typing to Hoopa. Hoopa can also opt to use Dark which would cover both its weaknesses.
Tera: Dark, Poison, Fighting, Electric, Fairy
Hoopa-Unbound is a power house set back by its typing. Having no resistances and massive Bug and Fairy weakness, not to mention its poor physical bulk. This makes Hoopa-U a great wallbreaker, but almost entirely useless against offensive teams, which will look like they'll be pretty dominant this gen. However, Hoopa is in luck with Terastallizing fixing its biggest issue in lack of resistances on top of making it more of a threat. Its spammable Dark moves can be made even stronger with Super STAB while also giving it a Ghost/Dark resist. Poison is good as it combats Bug/Fairy while giving STAB to Gunk Shot. Fighting is similar to its Confined form but with physical options as well, and same with Fairy. Electric makes its Thunderbolt pretty devastating too.
Tera: Ground, Water, Fire
Ground on Volcanion is by far the best option due to its unique typing and Water Absorb. Volcanion's weaknesses include Rock and Electric, both of which Ground resists, Ground is only weak to Grass and Ice thanks to Water Absorb, and it also has Earthpower. Water and Fire are fine options too, even if it does lose some defensive utility.
Tera: Just see Girafarig
Literally just see Girafarig
Tera: Dark, Poison
Same as most other Mono-Fighting types
Tera: Grass, Rock
Seeing all the nerfs to Rillaboom and its movepool, it got hit a lot and can't do as much with Tera besides making its Wood Hammer nuclear or giving itself good Rock coverage with Tera Blast.
Tera: Fire, Fighting, Poison, Dark
Cinderace is a very unique Terastallizer because Libero got nerfed so hard that it arguably could be worse than on non-Choice sets. Like if you want to use its new Sword Dance move, you will become a Normal type and lose your STAB on even Pyro Ball, Gunk Shot, and High Jump Kick. You could honestly just run Blaze and Terastallize pretending it still has old Libero. Oh, and the new Fire Starter, Skeledirge, would otherwise counter Cinderace and threaten to burn it with Will-o-Wisp when it uses Sucker Punch. They really want to kill this wabbit huh?
Tera: Water, Ice, Dark, Ground
Really generic ass Mono water type who doesn't really get much use out of Tera. At least its fellow starters were nerfed so hard that it'll see them in the lower tiers.
Tera: Fighting, Fairy, Dark
You might see a heavily nerfed Zacian, which it is, but while it can only activate Intrepid Sword once, it can Terastallize and give itself STAB on Close Combat, which is pretty huge, as well as make Play Rough and Crunch stronger. Like with Magearna, Zacian-C already has an amazing typing so it doesn't get much of Terastallizing besides STABs.
Tera: Flying, Fighting, Water
Ok what the actual fuck. Did they really need to nerf Zamazenta? Sure it gets Stone Edge and Body Press, but they nerfed Dauntless Shield and its defenses. Honestly don't know what to do with it either since it Terastallizing either focuses on Offense or Defense, not both at the same time like most Pokemon and it doesn't do that great in either department.
Well at least it can easily be OU now with how much powercreep their is and it being nerfed too. Defensive teams might actually have a backbone if this thing is unbanned.
Tera: Steel
so Eternatus is still a great Pokemon, but like with most great Pokemon it doesn't need to Terastallize as often since it already has a great type and coverage. The typing and its bulk is what makes it an Ubers Staple. But if you do have to Terastallize with Eternatus, use Steel
Tera: Steel, Poison
Despite the nerf to Wicked Blow (wow a whole 5 BP loss), Urshifu is definitely way more of a threat to defensive teams now that it can cover its Fairy weakness defensively and offensively. After a bunch of power creep is banned, Urshifu will likely go too, but who knows since this Meta seems way more offensive.
Tera: Electric, Water, Fighting
I actually talked about Urshifu before, like with Heatran, but way way back with all the Gen 8 OU mons. Not much has changed besides the complete removal of Ferrothorn, Toxapex being nerfed into oblivion, and Slowbro losing Teleport and Scald. So yeah it definitely got massively indirectly buffed in that regard. Additionally also getting Protective Pads but better too. Oh and before I originally thought you'd have only 1 STAB with Tera too. Anyways the types it uses are pretty self explanatory. Electric for a boost to Thunder Punch, and Water or Fighting to make it's STABs hit really hard.
Tera: Steel, Fighting
You know, Zarude's typing is a little under appreciated offensively and defensively. Sure it has a bunch of weaknesses, but it also has a bunch of good resistances and is complimented well by Steel as it resists a whole bunch of things Zarude is weak to. Fighting can be used for Close Combat STAB, but Fighting overlaps a lot with Zarude's weaknesses.
Tera: Grass, Ice, Electric
Regieleki is honestly overhyped with Tera imo. It now gets coverage besides crappy low BP moves and Normal, but doing so means Regieleki, an incredibly frail Pokemon, hogs the Tera spot for all match-ups. It does mean most Ground types aren't switch-ins, but you also make Regieleki part Ice when you do, and if you use Grass with some defensive utility, you are still walled heavily by Dragons and some of the common Ground types like Landorus-T. It also doesn't help that immunity abilities like Volt Absorb are going to be pretty common since they pair so well with Tera. You can also Ungabunga and make Regieleki hit really hard with Electric Tera.
Tera: Poison, Fighting, Fire, Dragon, Steel, Ground
While Regidrago won't be a great as Regieleki in this meta, it will definitely use Terastallizing better thanks to its movepool and immunity abilities being more in its favor. That is to say only Fairy is immune to Dragon and the things that can cover for Dragon's weaknesses are at worst unable to hit Flash Fire. Additionally it just has more options for STAB as well. It having a massive HP stat can also let it Terastallize into a more defensive typing and allow it to set up more Dragon Dances if you want.
Tera: Just see Avalugg
Being mono Ice, only difference between it and Avalugg in what to use for Tera is just using the more offensive types like Fighting or Ground, while it still can use the Defensive types to fix its typing issue.
Tera: Dark, Fairy, Fighting
Spectrier is in a similar category of getting coverage that it really shouldn't have. Being a strong Ghost type, it can now have Fighting/Fairy coverage that would also defend against Sucker Punch, so it doesn't even need to use cheesy sets this time around.
Tera: Grass, Psychic, Steel
When in doubt, just use STABs or Steel type when Terastallizing. Calyrex does at least use Steel pretty well given its movepool with Leech Seed, so it benefits greatly in the lower tiers
Tera: Fairy, Fighting
Welp, AG for this thing. Like with Spectrier and Dragapult, you probably shouldn't give something like this coverage that it shouldn't have AND also lets it resist Sucker Punch.
Tera: Steel, Fighting, Ground, Water, Ice
Calyrex-I is a great abuser of Terastallizing as it can fix its typing weaknesses when it needs to while providing great STAB options for some. It can spec more defensively too since it has regular Calyrex's moves like Leech Seed and with it having nearly the same bulk as Lugia. It'll also benefit from its better form being AG so it can actually be used itself. You can use Ice, but Glacial Lance is not needing power with how strong it is.