- Ground
- Water
- Fairy
- Dark
1 - Obviously Primal Groudon, has had significant influence in both defensive and offensive building as a whole and has been the metagame's best role compressor period. Pretty much everything can be said about this mon as positive except for its lackluster speed which has been shown to be a mild issue against offensive matchups but in every other case this mon will always do something, no matter what the set (as long as it's listed as viable). Pretty obvious #1 for me here.
2 - Necrozma-DM, basically controlled USM alongside Primal Groudon for very similar reasons - its role compression was amazing, its offensive sets had little counters, and it singlehandedly allowed Primal Groudon to be fully unleashed with more offensive sets like Eruption SD and Mixed RP a lot more often.
3 - Yveltal, also had major influence in USM but this time it also has major influence in SM as being one of the major reasons why mons like Lunala didn't shine as brightly in the later parts of SM. Again, very much similar reasons to the rest of these other than it couldn't really sweep which is why I didn't put this in Duskmane's slot despite its continued consistency all throughout the generation.
4 - Marshadow.
This mon's influence on SM was severely undermined in the rankings. Quite possibly the biggest reason SM was a fairly dreary balance tier was because of this mon - it absolutely required multiple checks, of which most teams could not afford, and its incredible synergy with Mega Gengar basically took storm to a lot of builds at the time, especially those including Lunala and Mega Lucario. Xerneas had a grip tight as hell on SM but in USM it wasn't really comparable to how Marshadow changed builds throughout the generation.#
5 - Xerneas, in its sheer influence alone on SM, otherwise this mon would have dropped to around #9 alongside Zygarde-C and Primal Kyogre. Xern has very few checks and the major constriction it had in SM was a big reason as to why some players outright villified the metagame and tours such as No Geomancy were created. It was also possibly one of the bigger reasons as to why Marshadow didn't completely dominate the offensive metagame as it was an excellent offensive check to it through Choice Scarf and a brilliant reason as to why it has continued to stay viable to this day.
6 - Gengar-Mega, even though it seems as if it should be higher I personally put this mon lower as its overall influence on builds wasn't as big as Xerneas or Marshadow dragging it down throughout SM. Offensive teams tend to manage to avoid this mon due to the mega turn but outside of that this thing has had a brilliant dominance on the USM metagame and has chopped and changed balance builds consistently. It only lands in this spot just because Marshadow and Xerneas were that constricting.
7 - Arceus-Ground. I had forgotten about this thing's SM dominance and the power of support variants alongside its SD sets - it was a big reason as to why this became S for a while and even now it's still making sure that teams double down on Ground resistances because of how dominant Primal Groudon is. Sadly SD sets are not as powerful this gen but Calm Mind has been sure to make non Ho-oh builds have difficulty staying alive for long.
8 - Necrozma-Ultra - even though it was largely dominant for a good while in USM it definitely hasn't had the SM history to be as strongly influential as Arceus-Ground, and a lot of the checks to it were also shared by Psychic types previously seen in the metagame. But it deserves this spot strongly for it being considered almost banworthy at a point in time.
9 - Zygarde-C: See above nomination for #9 titan.
10 - Lunala - This mon truely formed the early SM and mid USM metas as one of the strongest and most painful wallbreakers to try and switch into, almost mandating Yveltal on teams as a result of its sheer power. Sadly its dip in the SM meta really did make this mon not as dominant as the first half of this list and the move towards other options such as Pursuit Marsh have crippled it, but for making these influences on teambuilding as much of a worry as they are this is above Primal Kyogre to me.
11 - Primal Kyogre - this mon mainly took off from defensive NDM and the major flaws it gave to team structures but aside from that its flaws are pretty evident in its major lack of offensive matchup. But for how much it has continued to change balance it deserves this spot and even now it can get a few shots at breaking down some of the lighter teams.
12 - Mega Salamence. As far as this mon has gone down since its dominance in ORAS, it has still made sure that along with Zygarde-C support Arceus stays dominant. Not really too much to say about it though, it keeps more of a low down in building as it is mainly lumped with Zygarde-C's checks.
13 - Arceus-Water: This is more about how it tended to draw out games in SM rather than its USM performance as USM Waterceus is quite lackluster with the lack of Ho-oh builds but it made sure to utilize the wide range of mons it checked well, and the more defensive Primal Groudon teams suffer from that.
14 - Arceus-Fairy: This has been largely eliminated by Mega Gengar but its absolute incredible compression outside of that makes it a wonder for a lot of teams in checking Yveltal and Marshadow, two major titans that can cause problems for a lot of teams. Recently it also joined the CM arc club and has become a significantly better punish to Mega Gengar and its innate resistances otherwise, continuing to cement more defensive variants on teams.
15 - Magearna. You may be a bit confused as to why you'd see an OU mon so high but this mon dragged out SM matches because of how well it checked Xerneas and Yveltal. It was a major boon to teams which absolutely needed checks to them because the selection was so thin. Then, USM rolled around and CM arcs gained popularity. For the influence this mon has on Ultra teams I put this in 15.
16 - Ho-oh - a big beater for modern day Arceus-Ground teams but this mon seriously struggled in the defensive Zygarde-C metagame of early SM and whilst it was prominent in later SM it continued to drop in USM as its lack of applicability to builds without heavy support somewhat made it irrelevant to some build types. It doesn't like the offensive metagame for sure, and it's basically on this list for its abuse of Arceus-Ground and Ferrothorn builds more than anything else.
17 - Arceus-Dark, CM has been quite significant for the SM days but this mon hadn't really come into its own until the modern day support variant was discovered, significantly increasing the viability of team builds against STAG and giving a global check to both Yveltal, Lunala and Zygarde-C really put a wrench in those sorts of teams.
18 - Arceus - Normal variants have been big all about but they kinda died off once Marshadow dropped and haven't really been massively prominent since. They've had a resurgence in USM but still have flaws and often times teams can beat it without changing up their build due to how well the top 4 check it. Strong mon that can give teams some trouble in the endgame but it's really no big influence at all other than its initial SM appearance.
19 - Ferrothorn. I think that whilst it's sorta dying down in the current metagame, it's always been hiding there and scaring out Primal Kyogre builds to the current day. As being the best Spiker, it has also seen relevance in enhancing Zygarde-Complete and its dominance over bulky offense. For that, it hits this 19th spot, but that doesn't mean it isn't heavily flawed right now.
20 - Mewtwo-Mega-Y: Pretty much follows all of these low submissions in that they put a wrench in builds for a short period of time abusing certain holes, but never got any further. This is the culmination of an offensive mon like that, only really making its case for the last few months of USM because of its difficult to switch into coverage, and the abuse of multiple Ultra builds which lack the sorts of hard resists that were somewhat common in SM. For that, it closes off this list.