Finally got around to doing this.
Garchomp
Typing: Dragon / Ground
Base Stats: 108 / 130 / 95 / 80 / 85 / 102
Ability: Sand Veil / Rough Skin
Garchomp remains one of the most dangerous threats present in the OU metagame. With bulk equalling that of Swampert, a massive base 130 Attack stat, and a decent 102 base Speed, Garchomp can switch in with ease and immediately pose a threat to the opponent. Though it misses out on Swords Dance, Swords Dance allows Garchomp to boost its already high Attack to monstrous levels. Add on its decent bulk which allows it to take the odd hit or two, along with excellent STAB attacks in Earthquake and Outrage, and Garchomp can run away with the game easily if it can set up. However, Garchomp's glaring 4x weakness to common Ice-type attacks, along with its only average base 102 Speed, allow faster Pokemon, such as Thundurus, Gengar, and Starmie, to destroy Garchomp with ease should they pack Hidden Power Ice or Ice Bea,. Bulky Steel-types such as Skarmory and Ferrothorn will walk all over Garchomp if it lacks a Fire-type attack, too. Although rare, Mamoswine and Weavile will also swiftly dispose of Garchomp with priority Ice Shard. Despite these flaws, Garchomp is one of the biggest threats in the metagame. If anyone is foolish enough to underestimate its offensive capabilities, rest assured, they will soon find out what a big mistake they made.
Magnezone
Typing: Steel / Electric
Base Stats: 70 / 70 / 115 / 130 / 95 / 60
Ability: Magnet Pull / Sturdy / Analytic
With the abundance of Steel-types in the metagame, it is no small wonder that Magnezone is such a common threat. Magnet Pull affords Magnezone the ability to trap any pesky Steel-type, and either KO it straightaway with its powerful STAB attacks, or set up on it with Charge Beam thanks to its wonderful physical bulk. Even though Magnezone might lack a large movepool, Hidden Power Fire and its STAB moves give it near-perfect coverage. Its huge base 130 Special Attack also makes up for its lack of coverage options. Magnezone is a very valuable Pokemon for teams which rely upon the removal of Steel-types for their attackers to sweep. Magnezone can also deal with the common Ferrothorn + Jellicent defensive core, making it a great choice for just about any team. Ironically, Magnezone itself is also a Steel-type, meaning it suffers from the same flaws that its Steel-type brethren have to deal with. A painful 4x weakness to common Ground-type attacks is probably the biggest, as Magnezone is not handling these attacks anytime despite its respectable 115 base Defense. Magnezone is also slow, meaning that speedy Fighting- and Fire-type Pokemon can KO it with ease. Despite these flaws, Magnezone can make unprepared teams pay, since it can remove entire defensive cores without blinking an eye.
Hydreigon
Typing: Dark / Dragon
Base Stats: 92 / 105 / 90 / 125 / 90 / 98
Ability: Levitate
A high base 125 Special Attack, decent defensive stats, along with a massive movepool and a wonderful ability in Levitate give Hydreigon a niche in the metagame, making it a very dangerous threat indeed. Hydreigon's wide movepool, which includes Fire Blast, Surf, Dark Pulse, Draco Meteor, and Focus Blast, allows it to check a large range of threats. Levitate and its decent defenses also give Hydreigon the ability to switch in with ease and take the odd hit or two. Hydreigon can also surprise its normal counters by running physical moves such as Earthquake and Outrage, which it is more than capable of thanks to its good base 105 Attack stat. Unfortunately, Hydreigon's odd base 98 Speed leaves it outsped by a large number of threats who can hit it hard with their STAB attacks, Dragon-types in particular. While Hydreigon's defenses are decent, they are not spectacular, and it can't stomach powerful attacks from the likes of Conkeldurr and Latios. It also has unfortunate weaknesses to common Fighting- and Bug-type attacks. As if those flaws weren't enough, special walls such as Blissey wall Hydreigon unless it runs Outrage. Still, Hydreigon is a dangerous threat who can raze unprepared teams easily if given the chance, thanks to its sheer offensive power and movepool.
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I think I let myself go a bit. I will look through the list again and see what else I can write.
EDIT: I am taking
Gengar and
Infernape. I will finish them off in a day or so.