Maokai - The Twisted Treant.
In this guide you will learn more about the two ways I play Maokai. An offensive way and a more defensive one. It is important to keep in mind that is guide is more about a tankish Maokai. I only tested them in 3v3 but they're suitable for 5v5 aswell.

Pros
-Very good support in team fights with his Ult.
-Good tank.
-Has the ability to scout the map.
-Easy to stay in lane early game with his passive.
-Good partner to lane with as you can pretty easily get the First Blood.

Cons
-Kinda slow.
-Poor escape.
-Harder to master than what riot says.
-Is affected by pollution =(

You might not like them but I've been playing with them on other chars and i think they suit well for Maokai too, I've tried it out and it gave some positive results.

For the ''offense'' build: I go with mana regen because you will soon have a deep mana pool anyway, magic penetration... well for the magic penetration, and flat cooldown reduction to improve the dmg output and the scouting ability aswell with the saplings. For the quintessences, I use flat health bonus to improve my survivability as a melee champion.

For the ''defense'' build: I go with magic resist since you already have a decent amount of armor from the items, and it helps you be more balanced. You could decide to go full armor or full magic resist depending on your opponents. Next I get flat mana runes because it gives you the opportunity to hold your Vengeful Maelstrom since you have less mana in that build. Then I choose magic penetration runes to improve my dmg output. Finaly I take falt health quintessences to increase my survivability as I play more as a tank with this build.

I recommend: Heal and Clairvoyance
Heal: Synergize with your ult as you can take a bigger amount of dmg and so deal a bigger amount of dmg.
Clairvoyance: Helps you avoid ganks and detect junglers. Also, if you play 3v3 you have the total control of the map if you combine it with your saplings.

*Others summoner spells could suits you best, thought these are the one I like to use.*

You can pretty easily stay in a lane with this item since it gives mana regen. You can also keep healing with your passive Sap Magic.
catalyst the protectorIt gives you nice amout of hp and mana early game + you can stay longer in the lane since it recovers alot when you lvl up.
Since maokai is slow or just rush your .
You're going to use every single stats on the item.
For the ap boost and the debuff.
Good cooldown reduce, armor, mana and even slow the attack speed of nearby foes.
Finish up with this for the ap boost, the mp regen and the cooldown reduce.
*This item is optional I just put it in my build because its my favorite optional.*

Optionals:
gurardian angellet you revive every 5mins (use this for the armor and magic resist or just if you die alot.)
Grab these boots if the other team has some heavy CC.
If you wanna bring more support to your team and cap your cooldown reduce.

arcane smash

Sap Magic is your passive ability, when people spam their spells aournd you it helps you keep healing trough the fight.
arcane smashArcane Smash is mainly used for the knock back and the slow.
Twisted Advance is a very great skill as it can be used to avoid taking dmg from other team's ults and root for a quite long duration great to keep people in your ult area, tower dive enemies or help your team get a kill.
Sapling Toss great dmg output, and also good to avoid ganks by scouting the map.
Vengeful Maelstrom is your ult, use it in every opportunity to svae teamates and use it even in 1v1 fight as it deal a decent amout of dmg and reduce the dmg you would take.

Skill combo:
-> ->arcane smash-> -> and keep spamming your saplings if they escape low hp.

Thats prety much it, I'd really apreciate feedback and comment on your down votes if you do so.

Enjoy~ :D