3 Mage, 4 Ocean, 3 Inferno, Amumu
Ocean + Mage or 6 inferno are just strong comp but not OP. But if you get Amumu and Mage’s cap or Cynder you can create an almost invincible comp with 8 units: 4 Ocean, 3 Mage, 3 inferno, 3 warden.
Amumu (1 ap, 2 tank items) can be the second carry and the protector (his stun) against assassins. Annie with mage cap can be the third carry.
The only way to beat this comp is tank Singed + 2 Poison.
Mage/inferno/ocean with Amumu (that replace Malph) are the most flexible comp. Because if you don’t get Cynder/Mage’s Cap then you can go 6 inferno or 4 mage+3 Mage+2 Mystic/4 Warden. Maybe that’s why players chose them.
Of course there’re more options such like Blender Noc, Khazix+summoners, Olaf, 2 Soulbound but these are less flexible.
Unfortunately rangers have a big issue to be viable: they walk into the enemies. I don’t know why Riot didn’t fix it.