As for megas, for now medicham metagross are good, mawile is better though. Scizor is nice, but not as good as he used too because of the tapus.
All of them are really good, i highly recommand having at least one in your team
If stall is decent (one team is actually good), the metagame is oriented towards bulky offense for now.
I'd say you have to get at least one counter to landorus (he's like in every team), to fairies (still op), ofc stealth rock, one wall breaker (to counter the stall team you can face).
Ofc, you'll need to build your team around a playstyle you like, just tell me if you have any favorite, to see how you'll have to build your team around it
Also, do you pref to play hyper offense/ bulky offense/ balance/ stall....? (plz don't answer stall)