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If you're looking to get into competitive:
1. Don't bother with the ORAS sets and prior. They won't be usable in standard tournaments very soon.
2. Buying singles is always the best way to build a deck.
3. If you're going to buy packs, shoot for Guardians Rising. It has a lot of cards that are good in most every deck.
4. Decks are usually 35 trainer cards, 15 Pokemon, and 10 energy. Sometimes fewer items for more Pokemon, but you don't need too much energy.
Big decks right now:
1. Gardevoir GX featuring Diancie
2. Metagross GX featuring Necrozma GX
3. Volcanion EX featuring Ho-oh GX and Turtonator GX
4. Greninja BREAK
5. Garbodor with Drampa GX, Espeon GX, or both
6. Darkrai GX
7. Tapu Bulu GX featuring Vikavolt
8. Tapu Koko GX (sometimes with Marshadow GX)
9. Golisopod GX
Other decks people are trying and "meme decks":
1. Alolan Muk GX with Salazzle
2. Tapu Koko with Weavile and Necrozma GX
3. Typhlosion
4. Umbreon GX
5. Alolan Ninetales GX
6. Lapras GX