Alrighty, lemme think here for a second. Threats and counters, threats and counters...
Not necessarily a threat, but it can be a real pain in the rump to KO if you're not ready for it. What's more is that you SHOULD be ready for it, as it's currently the most used OU Pokemon. Basic counters are Fire-types (Notably Volcarona for resisting STAB and 90% of coverage moves) Fighting-types (Blaziken, Scrafty, and Lucario, for flexible attacking or bulk), other Steel-types can also help. The biggest Ferrothorn Counter is definitely Heatran, as it's x4 resistant to both Grass and Steel, and also shrugs off setup with Taunt.
An annoying little bugger that spits in your face with bolstered priority. It's like a Linoone on serious 'roids. The standard Conkeldurr usually has Mach Punch, Payback, Stone Edge, and Earthquake, so Skarmory is a fair counter to Conkeldurr. Sigilyph could cripple it with a burn, and then hit back with Psychic. The most notable counter is definitely Ghost-types not named Chandelure, such as Dusclops, Cofagrigus, and Gengar. All three are immuned to the priority, which is Conkeldurr's claim to fame. Gengar stands out form the rest for offensive prowess, as one Psychic would likely OHKO Conkeldurr after some kind of passive or setup damage.
Yep, they're back, and they're hungry. Haxorus is the new arrival (Hydreigon is Uber, apparently?), and its not something to screw around with. Being a pure Dragon-type, it has a less exploitable weakness, but it also has far less bulk and Speed, meaning it's gonna be going last a lot, or it's going to set up stuff. A faster Dragon-type with good STAB such as Salamence, Dragonite, and Garchomp can easily wipe him off the map. Ice-types are less recommended, but a Water-type with an Ice-type attack is good, too; preferably one that resists some of its coverage coverage, such as Empoleon, Ludicolo, and Swampert. It's not too big of a threat, but if it gets a chance to set up, well, you'd better hope you have a dedicated wall on your team.
It's basically common sense, but this thing will set up the furst turn or two, then sweep. Even more common sense, though, is that Stone Edge will KO it without a problem. Just get a beefy Pokemon with Stone Edge or Rock Slide in there, and Volcarona is a free KO. Conkeldurr, Garchomp, Gigalith, Rhydon, Excadrill, and basically any other Pokemon that can learn a Rock-type move will make Volcarona bait.
A Pokemon that's painfully predictable, and has some easy weaknesses. Faster Pokemon with Earthquake such as Garchomp and Blaziken can KO it easily. Pokemon with Water-type STAB, such as Starmie, Swampert, Gyarados, and Feraligatr can also easily kill it off.
If it resists Leaf Storm, then Serperior is finished. Chandelure is the perfect counter, as it traps it with Shadow Tag, then picks it off with Fire-type STAB. Heatran, Blaziken, Houndoom, and any other Fire-type also doom Serperior. Powerful Bug-types such as Volcarona, Durant, and Galvantula also put him at bay. It's speedy though, so watch out. If it get a few Leaf Storm boosts, be very hesitant toward switching in Water, Ground, and Rock-types, as Serperior is likely packing a Choice Specs and won't stop Leaf Storm-ing.
A tough opponent, as you can't really regularly counter it with Shadow Tag. However, it has a lot of weaknesses and only average bulk, so an offensively inclined Pomemon would likely KO it, as it's also pretty slow. Pokemon with Water, Ground, Ghost, Dark, or Rock attacks will pick off Chandelure like nothing. Luckily, these are are the most used types in OU, so Chandelure's useless if it's picked off early.
A tough, unpredictable opponent with a plain set of resistances and weaknesses. Scrafty can be deadly if it sets up Bulk up, but thta only means that Priority is a huge weakness for it. Mach Punch abusers like Infernape, Conkeldurr, Breloom, and other priority users like Linoone and Lucario will likely kill it off in a few hits. General Fighting-types like Meinshao, Hitmonlee, Blaziken, and Sawk/Throh could also have a chance at KO'ing it. offensive Flying-types such as Staraptor and Archeops could also pick it off rather easily. It's also fairly slow and has little HP, so it won't last too long if you start attacking the moment it enters battle.
These are basically all of the major attackers in OU, but if you need any more of them, I'm sure others will add on. Have fun putting together a great team!