In competitive Pokemon cores can essentially be defined as two or more Pokemon that match up well as they both can take on each others weaknesses with relative ease. And in RU there are a few obvious ones that everyone has encountered numerous times such as Steelix and Slowking or Tangrowth and Slowking. Cores are what makes a team so stable and keeps it all together, but these are only two examples and this is where you come in!
Think, who are some of your favorite Pokemon are RU.
Who pairs well with them.
Post your cores and how and why they are so effective in the current metagame!
Note: Don't forget Nidoqueen and Escavalier dropped into RU.