The Rage Yeti Build
With the newest Surrender at 20 patch update, we can see that Riot is testing some interesting buffs for Nunu next patch. The buffs they're testing include:
"Blood Boil (W) - New Effect - Attacking enemies Enrages nearby Blood Boiled allies, increasing attack speed by X% per hit, up to 40%"
So now, Nunu can go wild and beat his opponents down for some serious damage while also supporting his allies. This brings us to the new Rage Yeti build (for fun only).
If you're good at Nunu you can already get away with building
, because honestly Nunu is pretty item independent. He's incredibly tanky on his own and his insane gold leads tend to mean you can build whatever you want and still crush your opponents before the mid game ends.
With this new change, though, Nunu gets rewarded for building attack speed so he can ramp his ally's attack speed buff up as fast as possible. So here's the new Rage Yeti build:

Now you can break your opponents in half with a decent mix of armor, MR, Health, AS, and on-hit.
And of course, remember that no matter how you team fight, your primary goal as Nunu is to take dragons and barons and tilt the shit out of the enemy jungler. Remember that Nunu is a Chef, and as a Chef he constantly tastes a dish and his reaction is always "Needs more salt".
Get cooking, summoners.