I'd like to add some new Teras that have been popping up more and more as of recently :
- Ho-Oh : Tera Flying. It's the main Tera on the Scarf set, adding power to its best offensive STAB while also removing some weaknesses is very good for such an offensive set.
- Eternatus : Tera Steel. While not being as popular as the other Teras, it has been used a bit more to help against other Eterns on a pinch, while also providing some usefulness against NDM, Zacian and Caly I (the last 2 being covered by Fire)
- Arceus : Tera Ghost. I feel like it's a staple of this Mon, but maybe I'm wrong. Tera Ghost being able to help against Korai and other E Killers, while also giving STAB to Shadow Claw, is always gonna be good.
- NDM : Tera Poison and Dark. Tera Poison is a staple on the double dance set, which is a pretty recent set but still. Tera Dark is a much more recent novelty, being used on the Defensive DD set to help against opposing NDM, Lunala, while also giving STAB to Knock Off.
- Lunala : Tera Fighting. While not being as prevalent as the other two Teras, it is still being used on the Focus Blast variant. It provides a Dark resistance (which helps a lot against Kingambit and K Off NDM), while also giving STAB to Focus Blast. The rocks resistance is a nice bonus.
- Arceus Ghost : Tera Fairy. I feel like Tera Fairy is always gonna get some value on these slower variants of Arceus, and it is always a good type to have.
- Gliscor : Tera Fairy. Pretty much the same reasoning as for Arceus Ghost. While not providing as much value as Water, Fairy is still a valuable option against Dragons.
- Pecharunt : Tera Fairy. Yes, I know, it's pretty much the same as the two before, but it's even more valuable for this Mon to help against the Korai 1 vs 1. Being able to still tank both its STABS, while also being at least neutral on its main coverage, and being immune to Scale Shot before lowering its offense with Parting Shot, is a lot of arguments going for it.
- Giratina : Tera Water. Not the primary Tera on it, but still useful, although not in the same context. While Tera Fairy helps against Korai, Tera Water mainly helps against Kyogre while also keeping a good defensive profile.
Now I'm gonna add some niche options that I don't think are strong enough to be put here, but still provide some value :
- Koraidon : Tera Fighting, Dragon and Ghost. The first two Teras are used on the Band set to help break anything that comes close to it (mainly Fighting to 2HKO Ho Oh), Dragon also being used on a non Loaded Dice set to help provide some consistency to Scale Shot. Tera Ghost is mainly here to help against E Killer, while also being a good surprise Tera in Korai 1 vs 1 if the opponent decides to go for Low Kick or CC.
- Zacian : Tera Grass and Dark. Both aren't played that much, Tera Grass being mainly played on the Trailblaze set, but Tera Dark has seen some popularity combined with Crunch, mainly to help against Lunala.
- Arceus Electric : Tera Flying and Electric. This Mon doesn't Tera much, but I feel like those two Teras are valuable options to consider ; Flying being more defensive (mainly to help against Ground spammers but can also help against Korai) and Electric being purely offensive (to grab some ranges on some Mons).
- Ribombee and Deoxys Speed : Tera Dark. I'm regrouping those two in the same category, which is mainly to help against Grimmsnarl HO's. It's very niche but can still be useful. "Why not Glimmora then ?" Because it doesn't need as much Prankster immunity as the other two.
- Hatterene : Tera Steel. While not really helping against Korai, it does a lot against Zacian and Etern. Mainly a surprise Tera tho.
So yeah, here are my suggestions. Thank you for reading.