The tier list no one asked for
There's a few ways to dominate Legend of the Poro King. Abusing To the King! when you have Poro King up is a great option to cheese the hell out of displacements. Making sure the enemy can't use their Poro King well by killing him quickly. OR the good old fashion popping off in teamfights, getting resets constantly, and just pentakilling everyone constantly.
Poro King Abusers
With an ally Poro King alive, Blitzcrank should be on the lookout for hook opportunities. Hitting a hook then immediately using To the King! brings Blitzcrank and the enemy for free cheese kill.
Same principle as Blitzcrank except with a target skill. Skarner rushes towards an enemy carry and uses Impale (R) into To the King! for a free kill.
Poro King Killers
With Kog'Maw Bio-Arcane Barrage (W) he shred's any tank with ease including the Poro King. On the flipside Kog'Maw with the help of a Poro King takes turrets down incredibly fast.
Blaze (Passive) will make quick work of Poro King. The detonation from your Blaze if you hit the stack requirement will cause havoc with how much visual clutter there already is in this mode.
Katarina can wait a bit before jumping into a teamfight, if Katarina can get one reset the fight is in her hands. Even if your team isn't as...skilled at getting snowballs to hit, winning every teamfight will still result in wins.
Weaker early game than Katarina, Kha'Zix excels at being able to assassinate the enemy carrys. Evolving R or E first is a great way to give Kha'Zix more options in fights. By level 11 Kha'Zix should have E evolved to make good use of Leap resets though.