What are tips for teambuilding in VGC Regulation M-A?

Question

Which Pokemon form good cores? What are the most used Pokemon, and how can you best stop them? Which teams are used a lot?

Comments

This is (hopefully) going to be a series of questions on the Champions formats as they come out, if Fizz doesn't close this one

---

Mega Charizard Y - Venusaur Sun teams seem pretty popular

---

Rain teams with Archaludon also seem highly used. I build most of my teams around doubles, because strategies greatly expand when 2 pokemon are out there.