Nominating:
Cobalion AKA Battle Soul
What effect did Cobalion have on the metagame?
While Cobalion didn't have a major period of dominance like a top 5 caliber titan, such as Moltres, Reuniclus, or Gligar, it was a face of RU offense starting around Stage 3 of XY RU all the way through the end of its tenure in ORAS. Even before then, Cobalion was a solid Pokemon thanks to its ability to check the likes of Sharpedo, lead Omastar, and Tyrantrum with its primary set. Stage 3 is when Cobalion really started to take off as its support set rose as an all-purpose lead for many offensive teams due to its strong matchup against Omastar and Sharpedo, two of the biggest threats on offensive teams at the time; the former for its hazard stack ability and the latter for its cleaning potential. Cobalion was one of the big reasons that Omastar's reign as the best offensive hazard stacker in the tier was short-lived as it instantly gave up significant momentum to Cobalion offense due to the combination of Taunt + Volt Switch. It also forced many a Sharpedo to run Hydro Pump over Waterfall, as most did not want to give up Ice Beam coverage on Grasses and Gligar, since Cobalion was one of very few offensive Pokemon that was not 2HKOed by a STAB move. While Cobalion's SD set was significantly worse with Doublade in the tier, it became a top sweeper option once Doublade left the tier. During the brief period Cobalion was in RU and Slowking was not, SD Cobalion was easily the most annoying sweeper to deal with, and forced the usage of the likes of Musharna on fatter teams to hold it down. However, even during its late days in ORAS RU, Cobalion was doing one incredibly important thing for offense, even with most Sharpedo at this point running special sets due to RU's lack of special walls; checking Tyrantrum. Perhaps the biggest reason Tyrantrum was not a massive threat during the earlier ORAS days was Cobalion and its raw physical bulk / typing. Once Cobalion left, it was like flipping a switch. Offense has never been the same or nearly as viable since Cobalion left as the playstyle lost its best all-purpose Stealth Rock user and / or one of its most potent sweepers. Keep in mind that, by the end of its tenure in RU, quite a few people were calling for a Cobalion suspect test due to the state of the meta (remember, no Slowking in Cobalion's final days) but it moved up by usage before a decision to suspect it or not was ever made
In what main roles was Cobalion used?
As alluded to already, Cobalion's most consistent (and arguably best) role was as an all-purpose Stealth Rock user. While the support set was primarily used on offense, it could also work very well on bulky offense and balance teams due to its strong synergy with AV Slowking and SpDef Gligar. What made Cobalion's support set so splashable was its movepool, bulk, typing, and Speed. Every team needs Stealth Rock, and Cobalion was easily the best SR user to go to if you wanted a momentum builder. Taunt + Volt Switch had nearly unrivaled momentum building capabilities and Cobalion's typing, bulk, and Speed just sealed the deal. Base 108 Speed made it faster than most other leads, its Steel / Fighting typing in combination with its solid physical bulk allowed it to pivot into key physical attackers like Sharpedo and Tyrantrum, which is something offensive teams almost never have the luxury of. Doublade's departure is what really drove Cobalion usage up as that freed up the Swords Dance set to be an incredibly threatening sweeper. While it couldn't take physical hits as well as the support set, Cobalion still had incredible physical bulk for sweeper standards, and could still survive revenge killing attempts from the likes of Sharpedo, Tyrantrum, and Hitmonlee on most occasions if it was healthy enough. An often-overlooked set for Cobalion was its Calm Mind set. While it would never really sweep on its own, its ability to lure in and severely damage / KO typical Fighting-type checks, such as Doublade, Gligar, and Slowking. Because of this, Cobalion could be paired with other powerful Fighting-types, primarily Hitmonlee and Virizion, as it could significantly wear down their best checks and counters. While Virizion could obviously do something similar, its Grass-typing meant that Gligar and Slowking were much less likely to switch into it, meaning it couldn't fulfill this role as effectively as Cobalion could.
What caused it to have a significant impact?
Simply put, Cobalion's typing, bulk, Speed, and movepool helped to make it the amazing Pokemon it was. It stood as one of the key faces of RU offense from Stage 3 all the way through the end of its tenure in ORAS, and was seen on almost any team that wasn't stall-based. Major threats, such as Sharpedo and Tyrantrum either fell in usage or were forced to alter their sets to have more consistent performance due to Cobalion's support set alone. While it held some threats down, it made some other Pokemon, like AV Slowking, SpDef Gligar, and Scarf Moltres, even better due to its type synergy with those Pokemon. Doublade leaving in early ORAS allowed Cobalion's Swords Dance set to really shine and allowed it to have a major offensive impact. While bulky Psychic-types, such as Reuniclus and Cresselia were nothing new in RU at the time, the threat of SD Cobalion forced the use of at least one of these on nearly every bulky team after Doublade's departure. Once these Psychic-types were banned and Slowking made a brief stint in UU, SD Cobalion was easily one of the defining forces in the metagame. While revenge killing many sweepers is as simple as using a Scarf user or some faster Pokemon with strong neutral coverage, Cobalion's typing and raw physical bulk made it more complicated than just "revenge kill with faster Pokemon that has neutral coverage with a STAB move". Faster physical attackers often needed a super effective move in order to revenge kill Cobalion, and RU has historically been a tier with limited fast special attackers. Even then, it's not like Cobalion has terrible special bulk, especially considering its only special-based weakness is Fire. Perhaps the most telling tale of its impact on RU as a whole is the state of offense. Cobalion's departure was also the departure of RU offense as we knew it. Tyrantrum instantly surpressed the playstyle and offense in RU has never really been as good since Cobalion left.
How do/did you deal with this Pokemon in RU?
Cobalion's support set and SD set were easily held in check by most bulky Psychic-types. Slowking, Cresselia, and Reuniclus were all solid ways of keeping it from having a major impact on the battle. While they were in the tier, Gligar and Doublade were good answers to each respective set. Cobalion's support set couldn't build momentum easily against Gligar, although preventing a Defog could be somewhat annoying for the Gligar user as well. Doublade was a cold stop to any variant of SD Cobalion, although the support set could slowly chip away at it with predicted Volt Switches. Faster special attackers, such as Whimsicott, Scarf Moltres, LO Accelgor, and Scarf Delphox could revenge kill Cobalion, although it should be noted that none of them could reliably switch in and Whimsicott would need chip damage and / or CC SpDef drops. Dugtrio could revenge kill some variants of Cobalion, however it would have a difficult time with Shuca Berry and Magnet Rise variants of the SD set while it was incapable of OHKOing the support set without some prior damage or a CC defense drop. It should be noted that the Calm Mind set could turn the tables on some Cobalion checks. These include Gligar, Doublade, and non-AV Slowking. Calm Mind sets were also slightly harder to revenge kill reliably due to the Special Defense boosts and raw physical bulk, but the need of Life Orb for power and Focus Blast as its only special Fighting-type STAB option made CM Cobalion much less consistent as a sweeper.