Assuming all these Pokemon are in SV, here is what I think gets to utilize the Tera Type mechanic the best.
Probably the best defensive user of the Tera mechanic that turns Avalugg from a crappy Ice type, into a Physical titan on par with the likes of Blissey physically, not only having similar bulk flipped over (as in it only takes Physical moves slightly worse than Blissey does for Special moves).
It can be Steel, Water, Fairy, Ground, Poison, Ghost, Dark, Psychic, Fire, Fighting, Grass, Psychic, ect. It’s like if you gave Multitype to Avalugg instrad of Arceus, but also can hold an item. What ever suits your team, which typically will be Steel, Ghost, or Poison if we are gonna be honest, Avalugg can be a valuable asset.
In OU, I see Avalugg being a staple pick on Stall teams now. Being physical Blissey that also has Rapid Spin and hits harder than a pool noodle, it would be very hard to pass up Avalugg. It’ll have its niche in other playstyles, similar to Blissey/Chansey would on more offensive teams, but less than on Stall. It still uses the Tera mechanic on itself, so plans of using a Pokemon built more around Tera Types is a not advised.
Also, honorable mentions
As I previously mentioned, one of the biggest applications is to use Tera typing on a sweeper.
What you would do is send out say Dragonite against something that is forced out against it. Dragon Dance while your opponent switches into something like Choice Band Mamoswine, then you Tera type into Water while they clicked on an Ice move, giving yourself another turn or 2 for free.
252+ Atk Choice Band Mamoswine Icicle Crash vs. 0 HP / 0 Def Multiscale Dragonite: 59-70 (18.2 - 21.6%) -- possible 5HKO
252+ Atk Choice Band Mamoswine Ice Shard vs. 0 HP / 0 Def Multiscale Dragonite: 28-33 (8.6 - 10.2%) -- possibly the worst move ever
You’re now have a Dragonite at pretty healthy HP, 2 extra free turns, and being at +2/+3 Atk/Spe. You also likely have Aqua Tail/Waterfall for STAB.
Additionally, you could be Steel, Ground for Earthquake, or a number of things for Dragonite.
Similar principle for Salamence and Volcarona.
Super STAB. No further comments.
Probably one of the most improved Pokemon and best users of Tera Type in OU, Zerarora combines all the ways you can use the mechanic to its fullest.
You can use it for making Zeraora into the preferred type the match, surprise the opponent with sudden type change, and Super STAB Plasma Fist. It has an expansive movepool with Close Combat, Knock Off, Play Rough, Acrobatics, and Grass Knot. You’re not just limited to giving yourself STAB or Super STAB, as a pivot you can make yourself Steel, which does lose some good STAB, but Zeraora become the most defining monotype in the games. You can also still make yourself other types for giving Zeraora more of a backbone other than Steel depending on your team’s needs or how common Fighting/Ground/Fire will become in SV (which just goes to show how defining Steel is).