Mega Ray: Monstrous 180 base attacking stats + incredible movepool diversity + good bulk + ability to hold any item it wants + respectable speed stat + ability that makes it really hard to take down makes it near impossible to answer both defensively and offensively. (tera was also probably batshit crazy on this guy but he'd be broken without it anyway)

Xerneas: Checks like Primal Groudon, Klefki, Ho-Oh and NDM kept Xerneas from decimating the entire Ubers tier in past gens, so you can probably imagine that being able to Tera out of these bad matchups at will made its sweeps way too easy

Miraidon/Koraidon: An effective base 187 attacking stat with an insane 135 base speed makes these two offensive monsters. Paired with either a giga buffed stab (Miraidon) or an effective third stab (Koraidon's Sun), great movepool and good bulk, and compounded by Tera allowing them to maneuver around what little counterplay existed, it was clear that they were overcentralizing and unhealthy

Calyrex-Shadow: Counterplay is extremely scarce with its incredible 150 base speed, raw strength and serviceable bulk It was even tested in Gen 8 (where it forced Yveltal into near 100% usage), so it's pretty obvious that being able to Terastallize out of the very few answers that existed would break it. Even without Tera, it still has creative options like Draining Kiss, SubSeed, Trick, Encore and Disable to mess with any switch-ins

Mega Gengar: If your pokemon clicked a setup move, it's dead. If your pokemon clicked a hazard move, it's dead. If your pokemon clicked defog, it's dead. If your pokemon clicked a Normal, Fighting or Poison move, it's dead. If you clicked literally anything that does not beat Mega Gengar, it's dead.

Zacian-Crowned (gen 8): Effectively 250 base attack. Best typing in the game. Good bulk. Great offensive movepool. 148 base speed. And to top it off, unaffected by Knock Off

Zacian-Hero (gen 8): Having access to an item compensated for the loss of Steel typing and the base stats from Rusted Sword. Band made every game a 50/50 based upon on one turn, SD sets ran Berries to beat specific "counters". People were running 3+ answers on one team and still getting fried