Hey there!
Teambuilding this season has become a whole lot easier and I can easily help you with this question by analyzing each restricted Pokemon:
Groudon
In previous formats, VenuKoal was very popular. Groudon has replaced Torkoal as a very bulky Pokemon who possesses a large presence which can not be ignored. There has also been a few trick room + gravity Duclops + Groudon teams, though not as common.
Groudon + Venusaur is one of the most popular cores in this format.
Zacian
Zacian tends to be a restricted which can be put to use in several teams. It has tremendous sweeping capabilities and it is also relatively bulky. Most teams tend to use Thundurus for its Defiant ability to scare off intimidators such as Incineroar and Landorus so far, and LapDog (Lapras + Zacian) has become increasingly strong. I forgot which tournament it was, but there was one where 4 of the top 6 teams were Zacian + Lapras. The idea is to set up Aurora Veil early game and get off Swords Dance in the late game to sweep the opponent's entire team. Lapras/Thundurus/Zacian is used by a lot of people as well.
Calyrex-S
Calyrex-S is also a really niche sweeper who has one of the best moves in the game, one of the best abilities, almost highest special attack stat and speed which outspeeds Zacian. It is often paired with Indeedee to either HH + Astral Barrage or emulate the Clefairy + Spectrier lead in series 7 by offering redirection.
Kyogre
Kyogre's water spout is scary if it isn't dealt with properly. TornOgre is a feared duo where the idea is to tailwind + water spout which dishes out a ton of damage. Teams also use Metagross/Kartana due to the better match up against a majority of the meta. Incineroar is also commonly used in TornOgre teams as a switch in if the opponent leads Rillaboom.
Calyrex-I
Calyrex-I + TR setter is good, not to mention that Calyrex-I can set trick room itself. However, Calyrex typically doesn't want to receive too much damage before dynamaxing. Mimikyu has also recently been a niche pick for Calyrex-I due to trick room + shadow sneak + will-o-wisp + taunt etc.. Dusclops and Porygon-2 are also used, and a second mode is also sometimes opted for.
Yveltal
There are 2 ways to run Yveltal (3 to be more exact):
1) Life Orb Yveltal.
2) Weakness Policy Yveltal
3) Assault Vest Yveltal.
LO Yveltal exerts immediate damage output (STAB + Dark Aura + Life Orb = x2.65 damage boost), Weakness Policy is normally chosen alongside Raichu for volt switching into itself/protecting Yveltal from Thundurus and Assault Vest is used for support. Yveltal + Metagross has been used lately, as seen with Jouex9's team.
Dialga
Dialga has a few choices as well. It can set up trick room itself or alongside screens, consistently dish out damage without having to fear being KOd. With Telepathy, some people use Dialga + Landorus to spam Earthquake but most Dialga teams also have Grimmsnarl. Watch out for Fake out + Trick Room as well.
Solgaleo
Solgaleo is the augmented Metagross. It's basically Metagross but faster and bulkier and a better STAB move. People use Spectrier + Solgaleo to bulldoze turn 1 or some people use TR Solgaleo as well. Solgaleo can also set up TR itself, but it typically doesn't want to take too much damage before dynamaxing.
They are the main ones you might come across, some quick other mentions:
Zyagrde - Zygarde + Set up is scary if it is successful (coil), CybertronVGC, James Baek and WolfeyVGC are all using it.
Eternatus - Recover is annoying and it is used as a bulky staller. I've seen Eternatus + toxic and personally hold a grudge against it...
Ho-oh - Ho-oh + Sand team is good for the typing match up coverage. People can use WP Ho-oh, LO Ho-oh or even AV Ho-oh. Lapras + Ho-oh late game perish song was also featured in James Baek's team.
Xerneas - not so common anymore due to the ubiquity of steel types. Tries to set up geomancy with Incineroar/Landorus/Amoongus/Indeedee.
That's mainly it for series 8, but the meta is still progressing. Here are some more teams:
Ho-oh:
Code: 0000 0008 MH3T 39
Paste: https://pokepast.es/29f18d2c1399e90c
Solgaleo:
Code: 0000 0003 H7DJ H4
Paste: https://pokepast.es/a167ce02d4493aa9
Kyogre:
Code: 0000 0001 YC4G M1
Paste: https://pokepast.es/05bf3b23eaf76a98
Zacian:
Code: 0000 0000 M96L N4
Paste: https://pokepast.es/ccb3c7521f1f9aae
Yveltal:
Code: 0000 0002 19P1 MM
Paste: https://pokepast.es/40cbbe70d84883ec
Ice Rider Calyrex:
Code: 0000 0005 VJ6H KW
Paste: https://pokepast.es/8b31899e878e0873
Shadow Rider Calyrex:
Code: 0000 0007 M7JG 1P
Paste: https://pokepast.es/357e371e42e7c83a
Xerneas:
Code: 0000 0008 DN1X H2
Paste: https://pokepast.es/e366193a1f952ea6
Groudon:
Code: 0000 0008 JK70 J2
Paste: https://pokepast.es/043a795349138567
Dialga:
Code: 0000 0008 NGD2 KL
Paste: https://pokepast.es/8437eb6e8050511e
Hope I helped!
