Welcome to the RU Core thread, where you can post, discuss, and debate about an essential part of teambuilding: Cores, and the synergy between certain Pokemon.
Effective cores are very important in competitive Pokemon, and any good player uses them, intentionally or not. Defensive cores rely on covering a lot of attacking types and common threats within the core, while also offering team support, so that the core is useful even when walled. On the other hand, offensive cores still like having the common attacking types covered between its members, but offensive synergy is most important. Be it one member of the core attracting Pokemon that another member can easily switch into and force out, one member luring and weakening the checks and counters of another, or one member being able to beat the Pokemon that the other can't, those are the characteristics that are wanted from a good offensive core. Finally, there are mixed or balanced cores, as well as support and pivoting cores, which blend in into offensive and defensive cores. Support or pivoting elements in cores aim to either provide easier switch-in and set up chances to offensive Pokemon, or help to wall and stall out the opponent in the case of defensive cores.
The point of the project is to help teambuilders, newcomers, and other battlers to assist them in helping building their teams. Therefore, please don't post bad examples such as Electivire + Pelipper. Post cores that truly work and be ready to defend why.
I suppose I should post a core as an example:

Yanmega @ Choice Specs
Ability: Tinted Lens
EVs: 252 SAtk / 4 Atk / 252 Spd
Rash Nature
- Bug Buzz
- Air Slash
- U-turn
- Tailwind/Giga Drain

Rhyperior @ Leftovers
Ability: Solid Rock
EVs: 252 Atk / 252 HP / 4 SDef
Adamant Nature
- Stealth Rock
- Earthquake
- Stone Edge
- Roar/Toxic
A nice balanced core that focuses on allowing Yanmega to kill as many things as possible. Like I just said, Yanmega is the star of the show, being able to destroy most Stall teams with its Choice Spec's boosted attacks. Unlike most wallbreakers, however, Yanmega even blasts through resists with its amazing ability of Tinted Lens. Bug Buzz is the main STAB, hitting hard and being able to destroy SpDef Psychic types, namely Cresselia, Slowking, and Meloetta. Air Slash, thanks to Tinted Lens, has nice coverage and is an overall nice second STAB with a cool chance to flinch. U-turn is a nice pivoting move, letting Yanmega get out of sticky situations it does not want to deal with. Tailwind is mainly filler, but can be used to assist teammates when Yanmega is on its last breath.
However, Stall still has a few answers to Yanmega, namely SpDef Registeel, AV Escavalier, Togetic, and Jellicent. This is where Rhyperior comes in. Since it beats all those mons, except Jellicent (which loses to Giga Drain Yanmega), this is where you use U-turn should your opponent have one of those mons. You then switch into Rhyperior, ready to destroy things. Stealth Rock is a move Yanmega absolutely loves, being able to 2HKO walls it couldn't before, such as SpDef Aromatisse. Rhypeior just happens to be one of the best users of the move. EdgeQuake is as amazing as ever, and easily beats the afformentioned switch-ins to Yanmega. Roar and Toxic are 2 last viable options. Roar lets you phaze out a switch-out, as Yanmega switch-ins will most likely switch out fearing Rhyp's STAB. It also lets you get nice hazard damage. Toxic is another cool move that just whitters down walls, which is always nice.
Although Rhyperior might not be thea most offensive mon, its amazing ability to support and help Yanmegs sweep is very nice, and that's why it works. This core would be very effective on balance teams that have trouble with Stall. So, why not go out and try it?
So that's just an example. There are many, MANY more out there, so why don't you take a minute and help the community out? Who knows, your core might even appear here :]
Core Archives:
Offensive Cores:
http://www.smogon.com/forums/threads/ru-cores.3506192/#post-5434258 Medicham/Doublade/Exploud core by astromund
http://www.smogon.com/forums/threads/ru-cores.3506192/#post-5434436 Whimsicott/Omastar core by Snotjoch
http://www.smogon.com/forums/threads/ru-cores.3506192/#post-5434684 Zoroark/Mismagius/Gothita core by Faded
http://www.smogon.com/forums/threads/ru-cores.3506192/#post-5436840 Swirlix/Dugtrio core by TRC.
http://www.smogon.com/forums/threads/ru-cores.3506192/#post-5436933 Zoroark/Emboar core by EonX-
http://www.smogon.com/forums/threads/ru-cores.3506192/#post-5438304 Mismagius/Reuniclus core by atomicllamas
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5441110 Leavanny/Kabutops/Moltres core by Chesnaught
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5441131 Durant/Meloetta-P core by Shaymin :D
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5441826
Tyrantrum/Escavalier/Hitmontop core by Rennyjesus
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5444960 Zoroark/Hitmonlee/Venomoth core by Silvershadow234
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5448215 Froslass/Zoroark core by Chenkovsky
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5448940 Combusken/Zoroark core by Peef Rimgar
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5449616 Exploud/Delphox core by Felony
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5453293 Doublade/Braviary/Leavanny core by astromund
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5472113 Moltres/Shrifty core by Arikado
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5510993 Vivillon/Lanturn core by GlassGlaceon
http://www.smogon.com/forums/threads/ru-cores.3506192/page-3#post-5580012 Cobalion/Zoroark core by Molk
https://www.smogon.com/forums/threads/ru-cores.3506192/page-3#post-5624216 Dugtrio/Meloetta/Yanmega core by Molk
https://www.smogon.com/forums/threads/ru-cores.3506192/page-3#post-5631385 Clawitzer/Meloetta/Virizion core by EonX
https://www.smogon.com/forums/threads/ru-cores.3506192/page-3#post-5652051 Meloetta-P/Cobalion core by Arikado
http://www.smogon.com/forums/threads/ru-cores.3506192/#post-5434436 Whimsicott/Omastar core by Snotjoch
http://www.smogon.com/forums/threads/ru-cores.3506192/#post-5434684 Zoroark/Mismagius/Gothita core by Faded
http://www.smogon.com/forums/threads/ru-cores.3506192/#post-5436840 Swirlix/Dugtrio core by TRC.
http://www.smogon.com/forums/threads/ru-cores.3506192/#post-5436933 Zoroark/Emboar core by EonX-
http://www.smogon.com/forums/threads/ru-cores.3506192/#post-5438304 Mismagius/Reuniclus core by atomicllamas
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5441110 Leavanny/Kabutops/Moltres core by Chesnaught
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5441131 Durant/Meloetta-P core by Shaymin :D
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5441826
Tyrantrum/Escavalier/Hitmontop core by Rennyjesus
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5444960 Zoroark/Hitmonlee/Venomoth core by Silvershadow234
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5448215 Froslass/Zoroark core by Chenkovsky
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5448940 Combusken/Zoroark core by Peef Rimgar
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5449616 Exploud/Delphox core by Felony
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5453293 Doublade/Braviary/Leavanny core by astromund
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5472113 Moltres/Shrifty core by Arikado
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5510993 Vivillon/Lanturn core by GlassGlaceon
http://www.smogon.com/forums/threads/ru-cores.3506192/page-3#post-5580012 Cobalion/Zoroark core by Molk
https://www.smogon.com/forums/threads/ru-cores.3506192/page-3#post-5624216 Dugtrio/Meloetta/Yanmega core by Molk
https://www.smogon.com/forums/threads/ru-cores.3506192/page-3#post-5631385 Clawitzer/Meloetta/Virizion core by EonX
https://www.smogon.com/forums/threads/ru-cores.3506192/page-3#post-5652051 Meloetta-P/Cobalion core by Arikado
Defensive Cores:
http://www.smogon.com/forums/threads/ru-cores.3506192/#post-5434296 Golbat/Gastrodon core by Turtleye
http://www.smogon.com/forums/threads/ru-cores.3506192/#post-5435896 Cresselia/Skuntank core by Jonater
http://www.smogon.com/forums/threads/ru-cores.3506192/#post-5437180 Tangrowth/Lanturn/Cofagrigus core by Rob.
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5463500 Cresselia/Cofagrigus/Cobalion core by srcphoenix
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5512781 Pelipper/Ferroseed/Regirock core by Sapphire.
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5570248 Cobalion/Aromatisse/Meloetta core by SixCatPower
http://www.smogon.com/forums/threads/ru-cores.3506192/#post-5435896 Cresselia/Skuntank core by Jonater
http://www.smogon.com/forums/threads/ru-cores.3506192/#post-5437180 Tangrowth/Lanturn/Cofagrigus core by Rob.
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5463500 Cresselia/Cofagrigus/Cobalion core by srcphoenix
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5512781 Pelipper/Ferroseed/Regirock core by Sapphire.
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5570248 Cobalion/Aromatisse/Meloetta core by SixCatPower
http://www.smogon.com/forums/threads/ru-cores.3506192/#post-5434129 Gastrodon/Rotom-Mow/Emboar core by DFB3636
http://www.smogon.com/forums/threads/ru-cores.3506192/#post-5434204 Amoongus/Slowking/Rhypeior core by galbia
http://www.smogon.com/forums/threads/ru-cores.3506192/#post-5434303 Escavalier/Aromatisse core by Billtodamax
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5439860 M-Abomasnow/Alomomola core by The Leprechaun
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5575581 Swellow/Magneton/Gligar core by Turtleye
https://www.smogon.com/forums/threads/ru-cores.3506192/page-3#post-5631385 Registeel/Aromatisse/Druddigon core by EonX
http://www.smogon.com/forums/threads/ru-cores.3506192/#post-5434204 Amoongus/Slowking/Rhypeior core by galbia
http://www.smogon.com/forums/threads/ru-cores.3506192/#post-5434303 Escavalier/Aromatisse core by Billtodamax
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5439860 M-Abomasnow/Alomomola core by The Leprechaun
http://www.smogon.com/forums/threads/ru-cores.3506192/page-2#post-5575581 Swellow/Magneton/Gligar core by Turtleye
https://www.smogon.com/forums/threads/ru-cores.3506192/page-3#post-5631385 Registeel/Aromatisse/Druddigon core by EonX
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