Hello! My name is Nolan, and I consider myself a Nunu-lover. Perhaps it's the Yetti that grunts as he runs around slowing his enemies, or maybe it's the fact that he's super easy to play yet still fun.
I am currently ranked at Gold IV on the NA server, nothing too impressive.
Anyways, I hope you enjoy how I play Nunu.

The runes I run are kind of on the expensive side if you do not have a lot of IP.

9 Attack Speed Marks:
I run these for early clear speed as Nunu does not have very outstanding AP ratios.

9 Armor Seals:
The armor is really nice for sustain in the jungle.

9 Scaling Magic Resist Glyphs:
I prefer to scaling magic resist because by the time you're level 8, you'll have the 12 (11.52) magic resist you would have had if you took flat magic resist.

3 Movement Speed Quintessence:
The movement speed quints are really nice to run for early movement speed and as a replacement for mobility boots.

I prefer to run 21 on utility because of the movement speed buffs I get with Wanderer and Fleet of Foot. I'll put my other 9 points on defense because Nunu doesn't really benefit as well on offensive mastery points. However, I am a Nunu to play as a personal peeling frontline for the ally attack damage carry, instead of tanking for everyone. If you would prefer to tank for your entire team, I'd recommend the 21 points to go towards defense and the other 9 on utility or offense if you're comfortable with it.

Itemization is key on Nunu. I really love to rush my Sightstone after clearing bot side camps and solo'ing dragon. You should have just enough for your Sightstone and maybe some extra gold for a potion as you will be out after dragon.
After Sightstone, you'll want to rush Cinderhulk. This is not optional. Nunu does not do very well with any other jungle item bu this one. After Cinderhulk, you'll need some boots. I prefer either Tabis or Merc's for their passives. After that, I prefer to build Righteous Glory. This will provide you some good health and mana, as well as an extra speed buff for you and your team. After Righteous Glory, you basically build depending on which enemy is an issue in the game. If they're mainly magic damage, I'd recommend Spirit Visage. If they're physical damage, I'd recommend Frozen Heart or Thornmail.