Maokai is a great tank capable of not only taking damage, but dealing a surprising amount in a short amount of time. With that being said, this build is used to make Maokai tanky while maintaining that marvelous capability to destroy champions with lower health pools. So, that's the goal, here's my guide showing how I build Maokai and how I choose to play him.

Maokai works best as a tank, however he is more than capable of fitting a tanky DPS role. This means that he is the guy who initiates the fight, throws out tons of CC, and causes chaos, the whole time dealing tons of damage.

Greater Mark of Magic Penetration- I chose these runes because they give Maokai a decent, cheap addition to his damage. But why not get another defensive rune? Well, the Greater Mark of Health only gives about 4 health, multiplied by 9 places equals ~36 extra health. This is enough to last an extra basic attack, or 3 minion attacks. Meanwhile, Greater Mark of Armor and Greater Mark of Magic Resist don't really give enough of a bonus to be worth the slots and ip.

Greater Seal of Armor- Basic armor seals. The only other real alternative I can think for these would be either Greater Seal of Evasion, or Greater Seal of Health. Once again, the health bonus isn't enough to really be worth it and dodge runes don't give enough bonus to make a huge difference unless you build dodge, which you wouldn't really do unless you play Jax.

Greater Glyph of Magic Resist-I choose magic resistance for this spot out of personal preference so that I'm little more resistant earlier in the game. You could also go a route to try and further his mana regeneration or cooldowns. These however, are personal preferences.

Greater Quintessence of Health- Good ol' health quints. There's not really another great replacement I can think of other than Greater Quintessence of Armor or Greater Quintessence of Magic Resist

Per level runes: I don't really like per level runes, especially for defense. I build it off of this logic:
A flat armor seal, tier 3, will get you 1.41 armor from level 1 to level 18. Now, compare this to per level armor seals. One per level seal will give you .15 armor per level. This equates to an equivalent armor value (1.5) at level 18. Which means that in the early game, when the metagame is going to play the biggest role, you're significantly weaker and later in the game you are just as strong. So, in the end, it's just better to take the flat seals so that you get the biggest bonus when you need it most.

I go for a standard tank mastery set, 0-21-9, taking damage reduction where I can in Defense and Haste as well as Meditation in Utility.

I take:
Teleport- This is a useful spell because it really improves Maokai's map mobility as well as helping him pick up fallen lanes better. He can also use it to teleport into a ward and get a gank with Twisted Advance in conjunction with your allies that are already in the lane.
Ghost- This really helps with Maokai's low movement speed. It can be used to chase, run away, and can make the process of getting back to lane go much quicker.
Some Other Acceptable Spells:
Flash- Flash is also a really good spell, no matter who you are. But the cooldown on it is really long and the range for it isn't long enough compared to Ghost. In the end, it all comes down to Maokai. He's too slow and the small blink from Flash doesn't come in handy as often as Ghost will.
Exhaust- This is a great offensive and defensive spell. Use it to ensure a kill, or save a teammate. Overall, a great option.
Cleanse- A good spell, but the invention of tenacity items (or the purchase of a Quicksilver Sash if you need it) severely cripples the previous effectiveness of this spell.
Clairvoyance- Very situational. More often than not, I find that a Ward does just as good a job since with CV you're blindfiring and you really need an alert more so than a check.

Note* Tanks are very situational builders and a good tank never has the same build twice in a row. This being said, it is relatively easy to build as one as long as you know what to ask, and what to do with your answer. Here's my thought process as I play:

Sapphire Crystal and 2 Health Potions- This is a useful item. It gives you more mana which translates to more pester in the early game. You Newb! You should always get Mana Regeneration over flat Mana! Well, you're right. Mana regeneration is always preferable to flat mana, but if you start with a Meki Pendant, then you can potentially build it into a Chalice of Harmony and never have to worry about mana again and pick up 30 magic resistance. OR, you can go ahead and buy the crystal, get some mana, and then, depending on the situation, build a Rod of Ages if you want a more offensive build, or even better, a Banshee's Veil for 50 magic resistance, a killer passive, and health and mana. So, in the short term, a Meki Pendant is better. In the larger scope of things though, a Sapphire Crystal is MUCH better.

Catalyst the Protector- This gives you some extra health and mana during the laning phase and extends your lane survivability by healing you every time you level up. This is a very important item because it not only keeps you in lane longer, but it allows you to branch out into either an off-tank/roamer build by getting a Rod of Ages, or play more of a tank role and get Banshee's Veil.

Boots- This is your first situational item. There are 3 questions to ask: 1) Is the enemy team mostly AD, or AP; and 2) Is the scariest person on their team an AD or an AP based character and 3) How much CC do they have? Depending on the answer to these questions, you will buy either Ninja Tabi to counter AD, or Mercury's Treads to counter AP or heavy CC.

Thornmail or Banshee's Veil- This next item is also situational. Analyze the other team and ask yourself these questions. 1) Is the other team mostly AP, or AD; and 2) Is there a particular character on their team that is getting fed and if so, what kind of damage are they doing? After asking these, decide which is better, and buy it.

Your next item- This is where it gets slightly complicated. By now, you should be relatively beefed up against one type of damage, or moderately defended against both types. So, the best thing to do is to remember the first few things. 1)Is there anyone getting fed, 2) Who is doing the most damage, 3) Is there anything I can do to help my team? This opens up several branches for purchases. If you decide that you still need to beef up against AD, buy Sunfire Aegis because this will give you even more armor, health, and a pretty cool passive. If you still need some magic resistance, buy an Abyssal Mask. This will give you a lot of magic resistance, and make magic attacks by your team more effective. Lots of enemy CC means that you get a Quicksilver Sash. Or, if the enemy team is relatively balanced; get items that give you all around benefits, like Guardian Angel.

Note* For a more off-tank/roamer build, getting items that benefit both offense and defense is a great idea. For instance, against AD, I would suggest Zhonya's Hourglass. Against AP, get Abyssal Mask. For health, get Sunfire Aegis or Rod of Ages

I try and balance out CC and damage. The way I go about doing this is to level up Twisted Advance to lengthen the root duration, level up Sapling Toss to dish out long range pester and damage, and finally, Arcane Smash because the damage output increases dramatically with each level up as well as gives a little bit of CC and an interrupt up close.
Maokai is a tank with the unique capability of being able to deal a large amount of burst damage. This isn't something that many people expect; so abuse it.
But there's more to skills than just knowing what order to build them. Knowing the most optimum way to use them is also important. So, here's a few ways that I use each skill.

This is a powerful ability that does an impressive amount of damage.
Using this to knock out waves of minions is very effective. By launching a Sapling Toss into the minions, then using Arcane Smash laterally into them is a very quick way to clear minions. However, this method should only be used if you are secure in your mana. This is another reason why Catalyst the Protector is a great item; by wiping minions, you level up faster, which then restores your mana.
Another important feature to Arcane Smash is the knock-back. This is often either overlooked, or overused. A knock-back is an interrupt. So, using it at its most effective moment would be to use it on champions that are channeling for an ability, like Nunu & Willump's Absolute Zero or Fiddlesticks' Crowstorm or Bountiful Harvest. Meanwhile, running toward an enemy just to perform the knock-back is a waste of time on your part. More often than not, the slow is suffecient.
Which brings me to that key part of this skill. The slow is very useful for its saving mechanism. Learning angles to intercept enemies slow the incoming enemy; resulting in your teammate getting away.

This is my favorite skill that Maokai has. It teleports him to the champion, which is a great initiator, and puts him in position to launch his combo.
A very important thing to remember though, is that it ONLY ROOTS, this means that this skill won't interrupt anything; it will however, keep them from moving, which is sometimes just as effective.
This is also a great saving tool, use it well.

This skill has nearly OP range. With it, you can stand safely out of turret range and bombard enemy champs who are hugging it.
However, there is a drawback; the Sapling AI is awful. So, while the range on it is simply too good to not abuse, it costs mana and is not strong enough to single hit minion waves at early levels. So, make sure that when you do use it, it hits the target that you were intending to hit.
The last thing to really note about this skill, is that it has the potential to deal damage twice (once upon impact and once upon exploding). This is a great way to maximize damage potential as well as heavily pick apart enemy squishies from a ways away if you're accurate.
Once again, WATCH YOUR MANA!

This is the skill that I take early, and max last. YOU NEWB!!! THIS IS YOUR ULTIMATE AND SHOULD BE LEVELED EVERY TIME THAT IT'S AVAILABLE TO LEVEL UP!!! Ok, I can see where you're coming from. 99% of the time, it is more beneficial to level up your ultimate, however on Maokai, the damage reduction on it stays the same, the radius stays the same, the cooldown is only marginally reduced (and really doesn't matter since you're only really going to use once every team fight anyway). so the only real benefit that you get from maxing this skill, is the nuking power that you get at the end, that is assuming that the teamfight stays inside the radius for the whole fight, none of your teammates die, none of theirs dies, the cap is hit, and you have the mana to sustain it long enough to hit the cap. More often than not, this doesn't happen. So, for every second of the teamfight not being inside of the radius is one more second that you can could have applied to the damage output from Sapling Toss or Arcane Smash or the root time of Twisted Advance.
That being said, the damage reduction is very useful and I personally use it most often in 1v1 or 2v1 scenarios to help balance the fight and maybe make them back off for long enough so I can get away, because who wants to fight inside a ring of death that just grows stronger the more they hurt me?
It's also an alright finisher. Not very often, there will be that one guy who manages to get away with half a bar of health and your Twisted Advance will be on cooldown. So, popping this in front and then immediately activating it will hit them with the threshold damage and will kill them.

So, thanks for reading my guide and I hope that you enjoyed it.
I also ask that you post any comments (positive or negative) below. I'll keep trying stuff out and keep an eye out for anything new and improved that happens.
In the meantime, please don't downvote the guide because you don't agree with one part of it.
Imo, Mobafire isn't about posting and saying, 'My guide is better than yours', it's about showing other people how to play and exchanging strategy with other players in order to get better.
So, if you do elect to downvote, please comment on the reason why you did so. That way I, and anyone else who looks at this guide, can see your point of view and we can all grow as players.
That being said, I wish you luck on the Rift and I sincerely hope that Gandalf comes out of nowhere and waves his staff, killing everything in front of it, and you end up winning even though no one else was shown to be slain. In other words, GLHF!