Conkeldurr has an amzing attack stat and solid enough bulk to survive multiple hits from a neutral attack. While its Speed is low, it can easily use this for either being part of a Trick Room team (which are very powerful with Gen V's slow Pokemon) or as a Pokemon to use to fight against an opponent's Trick Room team. Mach Punch is incredibly powerful priority, especially when you have Guts and a Bulk Up boost or two under your belt. And Guts activates fairly often, seeing as Toxic and Will-o-wisp are thrown around a lot. Drain Punch hits hard and heals up Conkeldurr, making it even harder to take down. And it can even use Hammer Arm to cause major damage while making it even better in Trick Room. Payback hits Ghost types hard, meaning only Cofagrigus can survive enough hits from it. Conkeldurr is easily the biggest threat, and can tear through the opponents team.
Best countered by: Cofagrigus (as long as your foe does not have to many Bulk Ups set up)
Mummy removes Guts, meaning that it won't get an attack boost by burn. While Payback does hurt, Cofagrigus has the defenses to take a few hits. Immunity to Fighting and Facade is always good to have as well, meaning you can switch this guy in, while your foe gets damaged by burn if it is already inflicted. It can also hit hard with Psychic if you have it.
Thundurus gets priority Nasty Plot, Thunder Wave, and Taunt. This makes it incredibly hard to counter along with incredible Speed and Special Attack. It gets enough type coverage moves such as Focus Blast, Dark Pulse, and Grass Knot so that very few Pokemon can resist its attacks, or it can just run HP Ice along with Thunderbolt. Thundurus hits incredibly hard, especially with Nasty Plot, making it the second biggest threat.
Best countered by: Reuniculus
Bulky enough to take a hit, hopefully. It is difficult to counter this guy, as it can set up priority Nasty Plot, and has priority Thunder Wave. Reuniculus can set up Trick Room, meaning Thunder Wave is only going to help, especially since Magic Guard prevents the chance of you not moving. Reuniculus is pretty bulky, and can hit hard with STAB Psychic, which should OHKO.
Volcarona can set up one Quiver dance, then sweep the foe with a few Fiery Dances. At that point, only priority will be able to take it out. It hits very hard, and with Quiver Dance is fairly well protected from special attacks. The Bug typing offers a nice Fighting resistance, and it can hit hard against fighting with Psychic.It is even a threat without Quiver Dance, as the Pokemon of Gen V are mainly lower than base 100.
Best countered by: Stealth Rock
Just set up Stealth Rock and you take away half its HP.
Hydreigon can take hits well, and hits incredibly hard with Draco Meteor, Dark Pulse, and Fire Blast/Flamethrower. While base Speed has always tended to make it used as a Choice Scarfer, the lower base Speed of the other Pokemon in Gen V tends to make Hydreigon outspeed its foes. It can also switch in on Earthquake.
Best countered by: Conkeldurr
Mach Punch. Really, nothing else needs to be said. Mach Punch just murders this uy coming from something as strong as Conkeldurr.
Cobalion gets in one Swords dance, and now you have an incredible sweeper with solid defenses. Justified is also nice, seeing as Hydreigon is so common. Cobalion can completely resist Dark Pulse, and then get a boost to its Attack stat that allows it to sweep the foe with ease.
Best countered by: Landorus
Landorus can hit Cobalion hard with STAB Earthquake/Earth Power (preferably Life Orb+Sheer Force boosted Earth Power), while also resisting Close Combat/Sacred sword.
Jellicent just cannot be taken down in one hit. This thing has awesome defensive stats and typing, along with decent enough offenses. This thing is very hard to counter, and can heal up by switching in on Water type attacks. Its immunity to Fighting is also a major point for this guy.
Best countered by: Ferrothorn
Ferrothorn resists Ghost and Water, gets a huge amount of healing with Leech Seed, and hits for a super-effective hit with Power Whip. It can also use the time to set up Spikes.
Braviary hits incredibly hard. Be it with Sheer Force+Life Orb boosted Rock Slide and Crush Claw, the wide type coverage encompassing Superpower, or STAB Brave Bird (Superpower and Brave Bird still boosted by Life Orb, although there is recoil) this guy hits incredibly hard. A major problem with it in ordinary battling is low Speed, however, the lower Speed of Pokemon in this generation means while Braviary isn't the fastest, it isn't going to be outsped every time.
Best countered by: Thundurus I guess. This guy hits hard but is easy to counter.
Thundurus outspeeds and hits with Thunderbolt.
An interesting thing to do is lead with this guy. Your opponent is likely to start with an entry hazard, so you can just use Sandstorm to either raise your Speed or more recommended raise the power of Earthquake and Rock Slide. This is because of the lower Speed thing. It can also then Rapid Spin away those entry hazards. And it still hits hard. STAB Earthquake always hits hard.
Best countered by: Conkeldurr
Mach Punch. Really powerful. Blah blah blah.
Carracosta's Aqua Jet is the main point here. Sturdy can aloow you to set up Shell Smash, then you can hit with Aqua Jet. I also like carrying Rest and Chesto Berry for when the main threat is taken down. Aqua Jet takes down plenty of Pokemon, and Sturdy gives you the chance to use it twice, which is enough to KO foes hit for super-effective damage.
Best countered by: Cincinno
Bullet Seed breaks through Sturdy and hits for x4 damage.
The lower Speed thing means Darmanitan doesn't need to run Choice Scarf, and can instead run Life Orb (or Choice Band). Flare Blitz is still going to hit incredibly hard, even without the Sun. Or yu can have Whimsicott set up the Sun and OHK everything that your opponent sends in your way. And it also has other type coverage attacks, so if you aren't using a Choice item, you can take down any threat.
Best countered by: Carracosta
Sturdy protects from OHKO, meaning two Aqua Jets should be enough to KO Darmanitan.
The lower Speed thing means that once again a hard hitting Pokemon is no longer the slowest thing out there. Outrage hits everything hard. Just use Outrage and you'll soon KO the foes entire team. Or Earthquake. I love using this guy on Eelektross.
A faster Dragon.
Ferrothorn is pretty obvious. It sets up your entry hazards, has Leech Seed, a great ability, very few weaknesses, plenty of resistances, immunity to Poison and Leech Seed, great defenses, solid enough attack for use with its high base power moves, and its low Speed even helps out Gyro Ball. Yeah, really powerful Pokemon
Specially based Fire damage, so you don't have to touch it.