[Mr. Maggot]

I think amumu is really hard to counter, with the new ap build that is going around.

Spectral wraith\golem(rarely), sunfire cape, Rylais, Liandras, merc treads and randuins/banshees for final item.
He just deals soo much damage yet remains extremely tanky and resistant to damage. He can easily zone out the adc, sometimes even kill him with his ult combo and a few seconds of despair.
Hes slightly weak early, but mid and especially late game you can be almost certain that you will win with amumu in your team.