Alright guys, we probably have/had tons of these threads but I will still start one.

I never played pokemon competitive but I finally want to try it out.
I know that I should have a pokemon with rapid spin or defog in my team.
I should also have a counter for mega kangaskhan and a counter for mega gengar in my team, probably even one for brave bird.
I have absolutely no idea which pokemon fit those roles and thats why I'm here asking you guys.
What pokemon can fulfill those roles and what other pokemon could I use for the other 2 slots?