It’s tough to summarize the whole meta in 1 post, I would check out the viability ranking resource for that.So I've never played National Dex's meta before, and I'd like to try and get into it!
Can someone give me a quick summary on what's good right now, what isn't, and key threats to think about using and looking out for in return?
As for threats to keep in mind while building, ash gren, mega mawile, mega charizard y, tapu koko, kartana, rillaboom, and heatran come to mind; however this is not a full list and you could check out some more resources if you want a more complete list.