Ammumu+Nunu
If Ammumu press R with Nunu in same time, the enemies cant jump out of Nunu's Ultimate... Mostly they die.
Blitzcrank+Nunu
Blitzcrank can grab enemies in Nunu's Ultimate. So they got the full damage too!
Thresh+Nunu
Thresh's Ultimate slows enemies, Nunus Ultimate too. So they got so hard slow, that they cant run out of Nunu's Ult.
In principle all Chamions with a stun, hard slow, grab, knoch-up or kick back!

With a Nunu you give your W the ADC of your Team, if he fights. You have to poke a lot with your E, so that your team can kill them easier! If you want to decide the fight, you have to trap them in a bush where you wait and than you only press R your Ultimate deals realy much damage.! It can kill 2 or 3 enemies instant if the ult gets full load! But its not easy to time the Ultimate. So you need some exercise.

Nunu is a champion with very strong sustain. You start skill your Q. At first you kill blue! Than you can go counter jungle, because Nunu deals a lot damage on monsters and Minions with his Q. So you can take enemy red easy and fast! You dont have to backport often like other junglers without that strong sustain! You can also go afk jungle if your team is good and no lane need help, because its realy easy to farm a lot with Nunu!

You can gank with Level 3 and you need the boots of Mobility! You activate your W than run on the lane and shot your E (ice-ball) on the enemy Champion! Than you shot your E again if the enemy try to run away. The kill have to take the laner! You cant do anymore than that and Normal attacks.