Maokai is quite honestly my favourite and most played champion in game, partly because he's a tree, but mostly cause he's a freaking amazing tree. When played right he can control the entire game, and be the main reason that your team just aced the enemy without losing a player. I mainly play tank champions, and none of them have the same massive amount of utility that Maokai has, which these days results in me picking nothing but Mao when tanking.

This guide will focus mainly on a solo top element of Maokai play, which allows him to be incredibly farmed, and to dominate any team fight as soon as he appears.

And if you're one of the people I regularly meet in solo queue who instantly rofl at the suggestion of solo top Maokai, honestly, go try it before you start that laughing fit!

Pros:

Awesome farmer.
Great solo lane choice.
Packs a large amount of damage without AP items.

Cons:

Often get flamed for being a "noob".
Have to put up with people insisting on going solo top and feeding.
Occasionally your team lets you initiate and decides to have a group nose picking session instead of following you in...

I don't think runes will make a massive difference to your game play as Maokai, but having played him through many many games, this is the rune set I've decided on. This allows me that extra beefy factor that allows me to stay in lane for ages without recalling, and still be a match for my opponent.
All of Moakai's abiltiies deal magic damage, so the Greater Mark of Magic Penetration are a no brainer, as that extra damage just makes a well timed combo even more scary for your opponent.
The Greater Seal of Vitality and Greater Glyph of Scaling Magic Resist runes do exactly what I want, as stated in the previous paragraph, allowing me to beef up, and not instantly get outclassed on my opponents first recall.
The quints are my standard Greater Quintessence of Health, as I find that the extra hp is useful on almost all of the characters I play.

Feel free to tell me about any combinations that you play with and prefer, as this is after all, only what I personally like.

Now, I get flamed enough for picking Maokai, and when I pick Heal and Clarity, I generally can't keep up with reading the comments. Maybe you're one of those flamers as well, but all I ask is instead of instantly judging and claiming "noob heal maokai gg", actually try it out! I wouldn't replace these spells for anything, as they continue to have use all through the game, and provide you with amazing lane staying power to begin with as well.

There are several reason why I won't play with any other spells on Maokai, and I'll try and explain:

In a 2v1 lane, which is generally the best option to play as Maokai, having these spells can make you a deceptively difficult to kill target. I often find that I can bait players on low / medium hp to overcommit to attacking me, only for Heal and Clarity to pop, and suddenly they're faced with a full combo attack, and much more health than they anticipated, which more times than not leads to me walking away with a double kill, and an increased level advantage due to undisturbed farming.

Heal has great synergy with your ultimate, and again can work as a bait, luring the enemy into staying and fighting in your ultimate, and receiving nothing more than getting aced for their efforts.

Combined Heal and Clarity can ensure that your team can sustain a push, allowing you to just do enough more to take that tower, or to fire off a few more combos, possibly landing an extra kill.

The masteries I take reflect these summoner spell choices, and help to increase my beefy factor.

I find that each game with Maokai is generally unique when it comes to my item build, as what works one game, simply won't be viable the next. I have a few items that I will always include in my build however, Mercury's Treads, Chalice of Harmony and Rod of Ages. These are all perfect items for Maokai, as they build up your tankiness, as well as benefiting your overall gameplay.

The choice of boots is a must. There simply is no other choice. As a tank you need to be able to move around freely, and since you've taken no move related summoner spells, boots with tenacity on are your only choice. This is hardly a bad thing, as no other boots are really that effective on Maokai.

Chalice is also a vitally important item to begin with, as it allows you to stay in lane indefinitely and has great synergy with your ult. And the extra MR never hurts.

RoA is a relatively recent choice for me, as it never featured in previous Maokai builds I ran with, but this was mainly to do with the large cost involved. However, this is why you want to bag yourself solo top! Maokai can farm like an absolute god, by rank 3 in your sapling you will be able to take out the caster minions with just that. Also, when playing Maokai solo top, you will often find enemy champions (on seeing you running with "such noob spells") will be all too eager to feed you kills. When this happens not only am I happy, but also hideously farmed. This is generally GG!

Once you have the core build made, what you get next depends entirely on the enemy make up. Items I use most regularly to fill my slots would be Randuin's Omen, Banshee's Veil and Frozen Heart. The focus on these items is to stack your defensive stats according to the enemy make up, and to get as much health as possible at the same time. This is all because of your passive! The more hp you have, the more your passive will heal you for, and the more defense you have, the less damage each enemy attack will deal to you. This goes especially for when you happen to be standing in your ult, which combined will often leave the enemy wasting abiltiies on you only to have your hp actually increasing, which I've been told is quite fustrating to fight against.
Of course, if the enemy was playing sensibly, they wouldn't normally waste ults / hard hitting abilities on the tank. However, the base damage from Maokai's abilties combined with your RoA and Magic Pen runes mean that they cannot afford to ignore you, as a Maokai is more than capable of iniating into your team, throwing down his ult, and happily bombing you with saplings taking a large chunk of your teams collective hp, which allows for an easy job for the rest of the team to clear up.

My end game builds dependent upon enemy team composition are as follows:

Balanced Team
Mercury's Treads Chalice of Harmony Rod of Ages Banshee's Veil Randuin's Omen and either Guardian Angel or Rylai's Crystal Scepter (dependent on if you want that extra bit of oomph for your abilties.

Caster Heavy Team
Mercury's Treads Chalice of Harmony Rod of Ages Banshee's Veil Randuin's Omen Force of Nature

AD Heavy Team
Mercury's Treads Chalice of Harmony Rod of Ages Randuin's Omen Frozen Heart Sunfire Aegis

Your amazing passive, this only gets stronger as you level, allowing you to take absurd amounts of damage in team fights late on in the game.
Arcane Smash A great short range disruptor, perfect for preventing escapes and a short duration CC to keep people within your ultimate.
Probably one of the best initating moves out there, although be aware of the possibility of following a Pantheon halfway across the map if he ultimates...
Your bread and butter for farming and harrassing, made even better by the fact they sound vaguely like kittens when thrown.
One of the best ultimates in the game - explained below.

As solo top, you need to max your Sapling Toss first, as this is your only reliable ranged farm tool, and a beast of a farming tool it is! By rank 3 you can take out the caster minions with one toss, and can finish off the melee minions with auto attacks, or Arcane Smash and Twisted Advance if you want to push (i.e. you just killed your lane opponents, to the tower!)

Saplings also prevent you being ganked, as placing them in the river whenever you are advancing the lane will give you plenty of time to react to the enemy jungler / MIA laners coming to try and chop you down.
Also, if you are lucky enough to find yourself vs 2 squishy champs, make the saplings count! They deal absurd damage, and if you place the original cast well, you can ensure that both damage elements of the sapling hit your intended target. So get used to guessing the moves of the enemy before you throw. This may take a bit of practise, but is well worth it, as the reward of pushing your enemies back out of experience range most minion waves will soon show when you invite them to try and kill you, just for them to gift you with a double kill.

As to the other skills, take Vengeful Maelstrom whenever you can, and try to max arcane smash and Twisted Advance equally, with a slight preference towards Twisted Advance. These are the skills that allow Maokai to be such an amazing tank, and to not need a move related summoner spell to initiate or escape (if that is ever needed D:)
When your team is looking to engage in a team fight, it should always be you who initiates, as you can regenerate vast amounts of hp, and can happily soak their initial damage. Wait until an opening presents itself, (i.e. one of their carries comes to the front of their group and is slightly over extended) then Twisted Advance into their face! In regards to what next, it depends entirely on how the enemy team react. If you have been doing your job and crushing top, they will most likely be scared of you, and will do one of two things. Either they will surrender that carry and run the hell away, or they will likely waste an ult or two on you, and try and end your killing spree. Either way this is a win win situation for you, as you are doing your job however they choose to react. If they decide to ult your face, make sure you don't panic, but instead make use of your passive, and hit stuff! Nom, ult negated! (Just make sure when you initate your team does follow you, otherwise you might actually die...)

Ult placement is something I hear a lot of people cry about in regards to Maokai, saying that his ult is awful, as all you have to do is run out of it, and it's effectively useless. However, this is just qq, as you hardly place your ult and just ask them to come step into it! You have to decide when it looks like they have commited to engaging, and have to also be careful about the right moment to detonate the stored damage as well. If the enemy are stupid enough to fight inside your ult after you place it, feel free to keep it up until either they are looking at escaping (in which case you trigger it for the damage component) or you feel you no longer need the damage reduction, and would quite like to take a large bite out of their hp.
Another slightly odd use of his ult is as a chase prevention measure. I have noticed in a lot of games that many teams will just stop chasing you when you place down your ult behind you, allowing your team to escape a situation that might otherwise have been horribly bad. This works especially well if you are escaping into bushes, say near the dragon in the river, and place your ult down just before you run into the bushes yourself. Strange, but often true!

Arcane smash is an amazing ability, but has to be correctly timed and positioned to have its max effectiveness. It can be used to regularly disrupt a team fight, to save a carry that is getting focused, and to prevent a low hp enemy from running away freely. Oddly, this skill allows you to pull off some interesting counters if you are fast (and lucky) enough. For instance, if you time it right, you can actually knock back a Jax who has targeted you with Leap Strike, stopping you taking damage, and preventing the gap between you being closed. This also works vs Tryndamere's Spinning Slash. Both of these requires amazing timing, and a fair element of luck as well, but is more than worth it when the enemy Jax starts to qq.

Both Twisted Advance and arcane smash can help you to escape ganks if you get caught my surprise. Use Twisted Advance to your advantage, and target the enemy closest to your tower. This allows you to instant get closer, and immobilize the enemy at the same time. After that, using arcane smash to knock back and slow when they get to close, and Twisted Advance to halt them again, generally ensures you survive. Depending how cocky they are as well, you can use they skills to kite them round your turret, picking up a few free kills (did this vs a trynd just the other day).

So overall, your job is to grab solo top, and farm like a mad man. You do need to learn when to be aggressive and when not to when in a 2v1 situation, but as a rule, it's best to let them push most of the way to your tower, and let them get complacent and cocky. Then when you decide to actually attack, armed with a 2-3 level advantage, they seldom see the damage coming.
If you can manage to farm reasonably well, you WILL carry the game, as the enemy will really struggle to deal with your presence in team fights.

Feel free to leave comments / suggestions, but please do try the guide before down rating!