Maokai is my personal tank of choice, he's up there with the strongest and most annoying tanks out there, and he's just all around fun to play.

Pros:
Extremely good tower pusher and lane pusher
Can take extreme punishment in team fights
Passive gives him outrageous staying power
Ult is fantastic in team fights and has a low cooldown
Root and knock back are a godsend for saving teammates

Cons:
No hard CC (stuns/taunts)
Can be hard to grab the attention of people in team fights
Not a very mobile champion (lol he's a tree what can you expect)

Marks: I get magic pen. marks because marks lack good tanky runes, and some extra damage on that sapling goes a long way. You can put whatever runes you want on reds, this is just my personal preference

Seals: Armor seals for early game armor, other choices are mana regen for his low mana pool and low mana regen, dodge, or health runes.

Glyphs: Magic resist glyphs, they help throughout the game, it's pretty much a no brainer, if you really want CDR that's your choice, otherwise I'd stick with MR glyphs.

Quints: I like HP quints but if you can't afford them, try to go with any tanky quints, if you don't have any, you could go for AP or whatever you prefer.

Pretty straightforward masteries, i like improved clarity so I'm more useful to my lanemate (if i have one) and my entire team late game.

Oh god the items as a tank, they're completely situational, judgement is the main factor here.

I've added another situational build, if you find your enemy has a lot of physical damage, go for build 2 over build 1, I'll be adding more of these over time.

Your core items will be:
,

The reason for these items is to get high health fast, and to get some magic resist along with some tenacity, which is always a great stat to have on your tank.

Some situational items that i find are helpful are:

, This is your auto attacking carries nightmare, you want to get this when the enemy team's AD carry is starting to get high damage and attack speed, because even with lifesteal, when they attack you, it's diminished and you have your team kill them easier.

, This is your other auto attacking carries nightmare, you get a lot of armor, you get good mana, cooldown reduction, and a really REALLY nice passive that slows the attack speed of enemies.

, This gives you extremely good health regeneration, good movement speed, and really good magic resist, an all around great item for when the enemy team has a lot of strong AP damage.

, Banshee's gives you some nice health, mana, and magic resist, and it's passive is just great, a free spell shield every 40ish seconds is welcome on anyone.

, This is essentially a cleanse that you can buy, with your tenacity from your Mercury's Treads you really shouldn't need this unless you're getting a ton of stuns and can't tank properly, this is probably my least bought item because i never find myself using it, but if you feel you need it, go for it.

, Okay this item gives good resistances, as well as a great passive that can help quite a bit, I like this item but i never get to it unfortunately.

, This item is one I don't usually get too often, but if you have a squishy team, I'd suggest you get it so they can get resistances out of the aura without wasting their money and inventory space to buy it.

I always ALWAYS rush Warmog's Armor, by the time you get it, your farm with your sapling is strong enough to take out caster minions with one sapling, which means 3 minion kills to add onto it's bonuses, after Mercury's Treads though it's all situational.

Some items i like to grab in certain situations

Fed Yi, Tryn, Cait, MF?
Go with a along with , hurt them and slow their attack speed as well if they decide you aren't a good target to focus.

Strong AP carries?
is great and grabbing a is just icing on the tank cake.

Lots of stuns?
I usually don't have to do this, but if it's to the point where it's ridiculous grab a

Those are the core tank items, they're the most expensive but they do their job extremely well for the price.

Your skill order should be R>E>W>Q

Passive, Sap Magic
It's an amazing passive, 5 spells and you get a % of your max HP back? sign me up! you might not notice it in teamfights much but it's keeping you healthy most of the time anyway.

Arcane Smash Q, Arcane Smash
I used to think this ability wasn't that great when i started Maokai, but I've learned since then, it's damage is awful, but the knockback is amazing, it's nothing like alistars Headbutt but it's enough when you Twisted Advance, Sapling Toss, and then Arcane Smash them, because by the time you've done all that, you've either done a ton of damage keeping them on your sapling, or you've let the teammate that's helping you get to them and start beating on them.

W, Twisted Advance
Oh the rage I've gotten for this ability, i use it, they flash, i follow, they get rooted and the team catches up, and kills them anyway. It's pretty straightforward other than that, try to nab chasing people with it or initiate with it and throw your sapling on them the second you can once you've rooted them.

E, Sapling Toss
First ALWAYS FIRST, it's the most amazing bush checking ability ever in the history of LoL. Okay I'm exaggerating but still, it's pretty damn good, it's got high damage if it hits both parts of the ability (the landing and the explosion) and it's pretty much the most annoying thing in the world for your enemies.

R, Vengeful Maelstrom
This ultimate is so underrated, you pop this at a tower and sit back and destroy it while the enemies do 20% less damage than their usual low amount on you, and you've just gotten yourself a turret, remember this does NOT lower turret damage, so you either initiate a fight on the enemies attacking you first under the tower so you get aggro, or you wait for the turret to die first. Also a fun trick I've learned with this ultimate, if you're chasing a low health enemy and they're out of your Twisted Advance range, you won't be able to get a sapling in because they fly too slow, but since your ultimate's range is so large, you can throw down the ultimate, and instantly cancel out of it so it hits the fleeing enemy, and should kill them.

I grab Teleport because when that Yi starts pushing a lane and everyone's focused on mid, you need SOMEONE to get to the lane before he kills the tower in seconds. Also Teleport is useful to get to a teamfight faster than you usually would be able to.

Clarity OMG you grab clarity what a noob! before you think that, hear me out, your ultimate costs a LOT of mana while it's up, it's not just a one time cost thing, it'll drain your mana, and if you want to be useful in teamfights, you want to be rooting their strongest person so they can't kill your squishies, and that costs a lot of mana with his low mana pool, clarity while you're channeling your ult is how you can make or break a teamfight by reducing damage for your team.

Yes midding maokai is viable, and in fact, if you play it right, very strong. I've made people rage quit middle lane as maokai every time i go mid, and the reason for this is:

They have no assistance! you have a distance closer, a knockback, and a strong early-level nuke, midding is a good choice as mao for his extremely strong early game presence, and by late game you usually have your team with you for most of the fights.

I will warn you, don't take mid over a person better suited for mid, and DO NOT TAKE MID IF YOU ARE NOT CONFIDENT IN YOUR MAOKAI SKILLS, you need to be 110% sure of yourself that you know what you're doing and you know your limits.

You should only be taking middle lane if your team doesn't have a good mid character, or if nobody else wants it on the team, because you're better suited for a solo top lane or a duo lane than mid over a carry.

Early game, you're pretty squishy for a tank, but your Sapling Toss is annoying as hell, and has a huge range and great harass, so you shouldn't have to worry about being harassed out of lane much. Also your passive's magic sap is super OP at keeping you in lane, so use it effectively, if the enemy is spamming abilities, make sure you get a hit off on a minion if they harass you, and gain that health back, DO NOT let it sit there on full stacks without using it unless you're at full health, or you have to B or else you'll die, but if you're smart that won't happen often.

Mid game, you should have a turret on your lane, or you should have at least kept your turret in tact, you should have warmog's and you should be attempting to stack it up while you're in lane. Try to save your teleport so you can jump to lanes when needed, and make sure if any other lanes need help and yours is fine at the moment that you teleport in, or walk if it's down, and help out your teammates, they'll thank you when you save them.

Late game, you're not invincible, trust me, don't try to rush into them alone, but you're pretty damn tough to take out, by this time your resistances should be high, and your health should be really nice, the only problem is your damage is now horrible, you do help with damage, but it's not much, and you can't take on anyone alone unless they're super squishy still. You can tank towers easily, you can push down towers with your ult without a sweat and you can run teamfights, you're pretty much a monster when you have a carry with you, because nobody can hurt you, and your ult is amazing for everyone but the enemy.

Your basic rotation is either:
Throw a sapling on a person and instantly twisted advance them, then arcane smash once the root wears off.

OR

Twisted advance, sapling instantly, and arcane smash when the root wears off, once you get your ult, you can twisted advance, and ult when you get there, or after you throw your sapling.

You should ALWAYS, ALWAYS throw your ult down in a teamfight, 20% reduced damage is nothing to scoff about, and it does damage at the end too, however small that damage may be, if you can get the kill with the ults damage to save a teammate that's almost dead, do it, don't worry about it, that teammate might get mad but you know deep down you saved him and that's all that matters.

Maokai is an awesome tank that can take huge damage, while dishing out great early game damage, and moderate late game damage, along with a strong laning phase, and strong lane pushing as well. He has a snare and a knockback, keeping people in place for the team for a good amount of time, and he's a really fun and unique champion to play. He's pretty much your tank that helps the entire team tank as well, and it is glorious.