Maokai is the best champ if you have ever played him and he is mostly overlooked. He has so much power and if you play him right you can change an entire game. He is also thought of to be a jungle or tank, but few people expect an ap mokai and when they do they'll be dead.

Pros:

is tanky all by himself
does massive damage with saplings
can easily change a team fight with ult
has a spell that can catch running enemies
is a freaking tree

Cons:

Very manna hungry (mostly early game)
people will get mad when you fail to be the team tank
ult can be easily avoided
can be burn by fire

For masteries I go with a usual ap route with 21 in ap offence, 6 on MR and armor, also I add less time being dead because sometimes you might be targeted.
*Note that this route can be change to your liking.

sapling toss is really good for damage, wards, and farming. These little grenades do massive damage on impact and when they explode which causes AOE damage which comes useful during team fights. Use these as wards as well, but sometimes its best just to buy wards. Main source of damage, but very difficult to spam early game.

Twisted advance can be used many different ways where it can be an escape device or a disabling device. Use this spell as a utility spell less than a damage one because even though it does good damage it has some disadvantages, such as an enemy flashing right on their tower being able to murder you. Also note that even though the spell doesn't let your target move they can still attack you so if you want to use it in a team fight use it when the enemy runs or before the battle starts to stop a tryndamere in his tracks.

arcane smash can be used as a source of damage, escape or to catch up to some one. This ability is good when starting a team fight since it will slow down the first champs causing your range carries to destroy them. It is also useful for when someone flashes next to you since using the spell up close causes them to jump into the air giving enough time to escape. Though please note that using arcane smash can sometimes free an enemy from your w.

Your ult is what makes you the person to start a teamfight. It can easily damage enemies who aren't smart enough to move and it reduce damage taken. I have seen times where my ult killes the entire enemy team once they finish us up. It is also on a short cooldown which make it a good spell to scare enemies into thinking that your team is hiding in a bush waiting for a teamfight.

For runes MP and manna regen both help during early game so you can do massive damage right away and also keep your manna full. The extra ability power lets you do some good damage once your opponents begin seeing your build and realizing you are killing them with your saplings.

For items I go all out ap damage. Getting rod of ages lets you gain back some manna and health every time you level up. Your boots lets you get some magic penetration and mejai's soulstealer lets you get some more ability power with assist and kills. Abyssal scepter reduces enemy MR while increasing your own. Zhonay's hourglass gives you some armor for carries and a cool ability to avoid all damage during team fights while your ult is up. Will of ancient will give you some spell vamp to be able to sustain your self and other ap champs on your team. If a member on your team bought will of the ancient and you feel like you don't really need it then go with a lich bane. Your death cap will give you the last amount of ap damage you need when a team start stacking MR.

Moakia is one of the most powerful champs and can easily destroy teams and while supporting his team. He is really fun to play as and has good damage and health. He is not too hard to get use to and can always cause teams to question their playing styles. People usually expect a tank Moakia and when he starts to sap their live it already done.