League of Legends (LoL) Question: Best runes for Nunu jungler ?

Marks - Greater Mark of Attack Speed: Increases jungle speed and brings Visionary up more quickly.

Seals - Greater Seal of Armor: Unnecessary for clearing camps with Nunu, but strong against minion aggro, champion autoattacks, physical damage abilities and tower aggro, especially in early game ganks and skirmishes. You could actually choose to replace these but there aren't many better alternatives. Greater Seal of Scaling Health, Greater Seal of Scaling Mana Regeneration and Greater Seal of Scaling Armor are decent options if you want a better mid-late game.

Glyphs - Greater Glyph of Scaling Magic Resist: Pretty standard on most junglers. Very efficient runes in terms of stats and allows you to mitigate magic damage without having to spend much gold on magic resist.

Quints - Greater Quintessence of Movement Speed or Greater Quintessence of Gold: Player's preference, really. MS quints give some nice utility, though you already have Bloodboil to haste yourself faster than most champions and most Nunu junglers go for Mobility Boots for roaming purposes. Gold quints give you more income since you want to spend a lot of time ganking and counterjungling rather than straight-up clearing camps. I prefer the Gold quints on Nunu, personally.

I like to run Greater Mark of Attack Speed to proc your passive Visionary more often. Greater Seal of Armor to defend yourself against the jungle creeps. Greater Glyph of Scaling Magic Resist because it will help you defend yourself against magic damage later on in the game.
For quintessences you can make a lot of things work however I like to run Greater Quintessence of Movement Speed for better ganks and just getting to places faster in general.

I'd suggest the defensive Runes as well. I don't find Nunu has mana problems so I'd even suggest defensive Marks as well. Nunu can't really do much other than waddle around and throw snowballs, as well as get the occasional perfect Ultimate (in the brushes is best since the enemies can't see it channeling that way.) Thus, defense is your best friend, but to each his own. The real challenge of Nunu jungle is knowing your capabilities, and that you're better off supporting your team and trying to combo your CC with Allies', rather than being the frontline warrior like other jungles.