Maokai is a tank/support-jungler that needs a lot of mobility so I took as first item the Boots of Speed. I am upgrading them to the Mercury's Treads because I think they are more useful in the endgame then the Boots of Mobility.
After the boots I take the golditems Philosopher's Stone and Heart of Gold, because as jungler it is very useful to take golditems and I can upgrade them to Shurelya's Reverie and Randuin's Omen useful items for Maokai, because you can use Shurelya's to get closer to the enemies and if u go in with ur stun and ur ultimate u use randuin's so that the enemies won't get away.
Aegis of the Legion is also a great item, because u need to support ur team as a tank that engages.
For more movement speed, magic resist and liferegeneration u must buy Force of Nature.
As the two last items i would buy Rylai's Cristal Scepter and Trinity Force to get some damage. Rylai's is so good, because u ever can slow them with ur saplings and then stun them again to get closer and focus someone out.