What effect did Charizard-Y have on the metagame?
Charizard-Y is the undisputed king of weather in Doubles OU. Charizard has consistently found itself in the upper-tiers of viability, a frequent contender for top 10 Pokemon in the metagame, and a Pokemon which many would not dispute as being a top-5 Mega. The greatest testament to Charizards dominance is that, in a metagame oversaturated with the likes of Heatran, Landorus-T, Thundurus, and Diancie, Charizard still finds itself, not just surviving, but rather, it's thriving.
Charizard itself has undergone very few changes in how it is used, the transition from XY to ORAS brought with it a bulky set to go along with the conventional fast attacker, but aside from that, the metagame around it has shifted. Teams have always had to find ways to switch in on it, and in kind, Charizard has adjusted. The conventional knowledge of bringing Heatran in on Charizard was met with Charizard running Hidden Power Ground, and now Heatran has to scout. Rock Slide was met with Charizard commonly pairing with Aegislash for Wide Guard support. If teams were not using Heatran, Dragon-type Pokemon became a near requirement in order to safely switch in on Charizard, and even then, most ran both due to the strain opposing Charizards put on board position. Landorus-I began using Rock Slide for Charizard-Y, Stone Edge was used instead of Rock Slide to get around Intimidate or Wide Guard combinations with Charizard. Various small subtleties in the metagame, things which players often overlook or have ingrained in to them, can be attributed to the existence of Charizard-Y.
In what main roles was Charizard-Y used?
Charizard-Y thrived as an offensive presence, relying heavily on its powerful Fire-type moves to carve out large chunks of the opposing team. The overbearing threat of a Sun-backed Heat Wave can often force opposing teams to scramble and reassess board position, and in many cases, determine which Pokemon to sack. Charizard also made a living by anchoring Sun-based offenses, such as the notable BIG Titties team, being the only viable Drought user in the metagame.
What caused it to have significant impact?
Having access to Drought allowed Charizard to define itself from the other staple Fire-type in Doubles OU, Heatran. With Drought, both of Charizards commonly used Fire-type moves, the spread move Heat Wave, and the single-target nuke, Overheat, became a constant threat to OHKO or near-OHKO every non-resistant Pokemon in the metagame, with all but the bulkiest Pokemon unable to withstand the hit.
How do you deal with this Pokemon in doubles?
Charizard is best countered with Rock-type moves, thankfully the existence of Rock Slide means that Charizard is kept in check relatively easily. Diancie, Landorus-T, and Terrakion are amongst the standouts. Other common ways of dealing with Charizard are with offensive Thundurus sets, and with Heatran, though Heatran is required to scout for the Hidden Power Ground variants. Faster Charizard spreads are also susceptible to being KO'd by stronger physical attacks, early in XY Doubles the Charizard vs Kangaskhan matchup came down to whichever one won the speed tie.
What effect did Mega Gardevoir have on the metagame?
Gardevoir comes out of Gen 6 Doubles OU as, arguably, the best Fairy-type Pokemon in the metagame. Earlier in the development of the metagame there were questions as to whether Sylveon was the better Hyper Voice user, the dominance of Azumarill backed by the plethora of "Bunny Rampage" clones, and the release of Mega Diancie all led to challenges to Gardevoirs status as the premiere Fairy-type. What Gardevoir provided over its contemporaries was flexibility. The conventional fast Gardevoir has been a staple of the metagame since XY was first released, and has adapted over time to the fluctuations and development of Doubles, more recently, Trick Room and middling speed varieties have come to the forefront of competitive play.
This flexibility and ability to redefine how the Pokemon is played is amongst Gardevoirs most uniquely powerful qualities. Early on in the metagame Gardevoir primarily ran Focus Blast in order to snipe Kangaskhan, later Hidden Power Ground became the fashion, others ran moves such as Ally Switch, Will-o-Wisp, Encore, Substitute, Imprison, and now, Trick Room. Opponents are required to respect all of these options when seeing Gardevoir in the Team Preview.
In what main roles was Mega Gardevoir used?
As mentioned, Gardevoir started off in Doubles OU as an offensive Pokemon, threatening the majority of the metagame with its Pixilate-backed Hyper Voice. It typically ran standard fast-sets with 2 STAB moves and a tertiary coverage option that patched any notable gaps in its team coverage. More recent variants now utilize Trick Room as a speed control option, though the fast-sets are still used to great effect. Middle speed Gardevoirs also exist, typically hovering around 263 Speed, though those are much less common.
What caused it to have significant impact?
Being able to combine such a potent offensive presence with its exceptional flexibility is what allowed Gardevoir to define itself over the course of 6th gen. The ability to anchor a team offensively due to its access to Pixilate-boosted Hyper Voices, or to characterize a team with Trick Room is something very select Pokemon are capable of, and to do so as effectively as Gardevoir can puts it in rarefied air.
How do you deal with this Pokemon in doubles?
Steel-types are the most surefire way to deal with Gardevoir. Aegislash, Bisharp, Heatran, and early on, Mawile, all worked as efficient ways to deal with Gardevoir. However, Mawile is currently non-existent in the Doubles metagame, Aegislash and Bisharp, are seeing less usage due to the release of Hoopa-U, and Heatran has to scout for Hidden Power Ground in order to safely tackle Gardevoir. Other methods involve the simple solution of using a powerful Fire-type Pokemon, such as Charizard Y, and opting for an Overheat, or a strong physical attack, such as a Return from Kangaskhan or Hyperspace Fury from Hoopa-U.