I'll reserve
Kyurem-B and Thundurus-I :]
Kyurem-B
Kyurem-B is a phenomenal wallbreaker Pokemon who can destroy Stall teams with ease. Kyurem-B has a monumental base 170 Attack stat, being tied for highest Attack in the game with Mega Garchomp. Kyurem-B also has a very large base 120 Special Attack stat backed by above average 125 / 100 / 90 bulk allowing it to take many hits. Ice Beam is Kyurem-B's main draw, as it has insane power and coverage when combined with Earth Power that allow it to take on Heatran reliably. Fairies may seem like a downfall of Kyurem-B, however Fusion Bolt 2HKOes all relevant Fairies. Where Kyurem-B falls short however is its not-so-outstanding base 95 Speed, which allows it to be outspeed by common threats such as Greninja and Thundurus, who can cause serious damage to Kyruem before it can even attack. Kyurem-B is still a huge threat with amazing coverage, and if you aren't prepared for it, then you might find yourself in trouble!
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Thundurus-I
Thundurus is a Pokemon so powerful it was deemed Ubers last generation, however this generation it is back in OU with a splash! Thundurus has an amazing ability: Prankster, which allows it to shoot off Priority Thunder Waves at set up sweepers and fast Pokemon in general. Thundurus has a powerful Thunderbolt and even learns Nasty Plot to make it immensely more powerful. Thundurus also has a rather large and expansive movepool such as Focus Blast, U-Turn, and Knock Off. Speaking of Knock Off, Thundurus can also run an excellent Physical set thanks to its other ability: Defiant, which allows it to switch into Defogs and get an instant +2 Attack. Some physical options include Superpower, Wild Charge, and the aforementioned Knock Off. Thundurus also sits at a very fast Speed tier of 111. Thundurus is certainly a huge threat in the current metagame that all teams should be prepared for!
Gary2346
Here's Thundurus :] Should I mention Stealth Rock somewhere in here, or would you leave it out?