As of right now, Miraidon and Koraidon are near unanimously considered as the two best mons in the tier with a slight edge to the electric motorcycle. The only ones who don't consider the motorcycles as the best mons in the tier are such a minority of the playerbase that the percentage is probably similar to the number of console fanboys on twitter. Compared to the previous box legendaries, the only one that really outdid them in their debut generation was both Zacian forms in gen eight. Other box legendaries were only good in their debut generations but Zacian was on another level with its always having a +1 attack boost along with its form where its biting a sword having the best offensive and defensive types in fairy and steel. However, both dogs were greatly nerfed when they only get their attack or defense boosts the first time they come in so they're much more managable now
In terms of ability, the thing about the motorcycles is that they directly benefit from their abilities because not only does it set sun and electric terrain, these effects also grant them a 30% boost to their main offensive stat. As for their signature moves, Miraidon uses its own quite a lot. It uses electro drift in nearly all its sets whether boots, specs, scarf, calm mind life orb, assault vest, whatever. The only time it doesn't use electro drift is on its bulky agility calm mind set with lefties. This is because those sets tend to favor parabolic charge for a bit of healing, unlike the life orb calm mind where it wants to deal as much damage as possible. For Koraidon, it almost never uses collision course for one simple reason, it learns close combat. Sure, cc's defense drops is exploitable but if it kills, you don't have to worry about retaliation and for priority, only extreme killer Arceus exists and that thing doesn't have a great time right now. There's also the option of low kick and with the obese legendaries, it is just as strong as close combat most of the time and it doesn't have the defense drops
In terms of game changing, the two of the warp the tier around them especially Miraidon. How? Simple, Ting Lu has such an incredible usage rate in ubers because it is one of the very few Pokemon that can temporarily stave off Miraidon. In fact, Miraidon often necessitates using a fairy type, either Arceus or Zacian, because if one solely relies on Ting Lu, well have fun eating all those draco meteors. Specs is probably the strongest or second strongest wall breaker in the tier with the only one capable of potentially surpassing its damage output is Kyogre's rain boosted full powered water spout. One might also think just use robo Donphan against the electric motorcylce since it's a hard counter as it can abuse electric terrain itself, immune to electric moves, and resists dragon, right, right? Yeah, no, one overheat and this thing is screwed in ways it never even thought was possible. That is why Ting Lu is the best option for it. That doesn't really stop Miraidon though since it can run any set it wants and one can't even use Clodsire since Miraidon gets taunt. Whatever the case, Miraidon is often hailed as the single best mon in the tier
As for Koraidon, part of the reason it isn't equal to Miraidon is of its typing. Sure, it gets a free stab on flare blitz but dragon and fighting gets walled by fairy Arceus. However, Koraidon can also run a number of sets. Band, scarf, sd flame charge, bulk up, a mix of flame charge and an attack boosting move, etc. Oh, and it also has taunt. Despite that, it also suffers from being relatively easy to check because aside from fairy Arceus, it is also walled by the very common Skeledirge, which is found on a lot of teams because it is the biggest middle finger to Zacian, so much so that the dog has to use tera ground or water tera blast to deal with the croc. Koraidon also suffers from the problem that all physical dragon types aside from Baxcalibur has for a long time, the lack of a good dragon stab. Dragon claw does piss poor damage and outrage is **** because that's like giving fairy Arceus or Zacian a free boost with cm or sd. Despite all these issues, Koraidon is still the second best mon in the tier after Miraidon for all the reasons just mentioned
One thing I have personally come to appreciate is that Koraidon, especially scarf sets, is very useful when in the low ladders on showdown because low ranked idiots love to abuse some dumbass last respects Basculegion with Kyogre BS. Koraidon takes away Kyogre's rain and with scarf, destroys Basculegion. However, at the end of the day, Koraidon is simply slightly easier to deal with which is why it is slightly behind Miraidon
As of right now, there are really only three Pokemon that were more broken for the uber metagame that they were banned. The first is obviously Mega Rayquaza in gen six and seven, both Zacian forms in gen eight, and right now, that ghost horseman thing whatever tf its called, in gen nine. That's the mon that that stupid one twenty base damage ghost move. I can't remember the names of either but the horseman, especially in a meta where it can turn into any type and the angel of death, Yveltal, not around, is too much. Miraidon and Koraidon may be the clear best mons in the current ubers and gamefreak obviously simped a lot for them as shown when they're given all the tools they could possibly need but as of right now, the uber playerbase doesn't find either as too much for the tier
