Mundo is the tank of Twisted Treeline and that is almost exclusively where I play him although he can do well in 5s. Other rivals may include Singed or Volibear due to their comparable ability to farm, tank, and still dish out damage. These are the three goals of this build.

1. FARM
2. Be the TANKiest SOB on the field (aka bait, initiate, tank turret)
3. Be worth your life in DAMAGE to your team. (Shen and Sejuani are good tanks, but no dmg output, aka useless in a 1v1 against a tryndamere for example)

Oh, and I know there is a lot of text, but I am not so pro at getting screenshots as I do not have a capturecard. So bare with my text descriptions.

PROS:

-No weak spots in game (early/mid/late)
-No need for damage items to still deal solid damage
-No MANA
-Can demolish other tanks with the %health dmg of the cleaver
-Good CC in DAT CLEAVER
-Can virtually stay in lane indefinitely with all his healing and regen.
-Large amount of dmg [135/sec] caused by AOE. Sunfire Cape and the W toggle. (Allows you to kill stealthed enemies, deal dmg while CCed and to just linger and deal dmg in teamfights)
-High Tenacity
-Hard to kill/catch with the super ult on low CD
-Really good farmer/Tower pusher when played and built correctly
-Can work alone with ease. (If you follow this build to its fullest, you will laugh at 3v1s as long as they don't have that much CC)
-Only reason not nerfed is because he can be hard to use (I guess?) cleaver can be hard to land and ult needs to be used at the right time.

CONS:

-Cannot solo a lane early game. Needs good DPS lane partner
-Sometimes has to big brother lane partner so they don't feed
-No burst potential
-Can be really predictably countered with dmg only coming from AOE and cleaver if enemies somewhat intelligent

This spell may be weak to some but perfect for Mundo. It serves two purposes:
1. Allows you a free heal in lane early game.
2. Allows you to heal that near death squishy dps at first blood or just in the lane.
3. Serves as a short buffer to prolong the popping of your ult. The uses of the ult will be discussed later, but all you need to know is "the later the better."
Exhaust is a no-brainer. Good escape and gank tool. Can shut down a dps or virtually stop a fleeing enemy.

Movement speed is HUGE in the TT, so getting this boost really helps especially when you are the tank and need to get around the field.
Almost all your damage is magic (Cleaver, AOE) so magic pen on both makes you deal a significantly proven increase in damage.
Greater Seal of Endurance These are new runes which I fell in love with when they came out. Mundo is all about high hp, and armor is generally not an issue with such great armor iteming for Mundo (will be addressed in Armor Troll Build).
Magic Resist is big. I find it easier to build armor then it is magic resist with the exceptions of the required Mercury's Treads and Spirit Visage. Also, people will try to counter you with Madred's Bloodrazor and the magic damage it deals so MR can be great for Mundo. Not to mention the way it gives casters a sad face when they sling spells at you and nothing happens as they sit in the back.

This is nice passive. Gives you an incentive to build high HP. But it is very useful early game when paired with the regen from Doran's Shield and Vigor to let you almost sit in the lane indefinitely.
DAT CLEAVER! the most OP thing about Mundo and your primary source for damage and CC(which is a flat 40% slow starting at level 1). This needs to be maxed first to lower CD and raise DMG. Cleaver is your bread and butter, I will go into more detail about DAT CLEAVER at a later time. Can be hard to master.
FARM TOOL This is your constant source of damage and when completely maxed will melt minion waves with ease. It is also good for killing stealthed opponents as you run next to them or for dealing damage while CCed or just being able to damage all enemies while on the run. It also gives you TENACITY while it is up on top of the merc treads. So keep it up while getting CCed for sure!!!!
Masochism is a very good ability. But it is not as important as the other 2(+ult). I primarily use it while TAKING DOWN TOWERS, but also for farming minions or jungle camps, or just for a lil extra damage.
I have another whole section for this ability due to its true power in the hands of a skilled user. But this ult is perfect when you have high HP and paired with Spirit Visage's CDR and regen boost, this ult is OP for sure.

EARLY GAME:
I start with Doran's Shield as the obvious tank item, but all 3 stats benefit Mundo. Health, check. Armor, check. HP regen, check. I will then sit in lane and wait for 825 gold (sometimes I'll back at 800) to buy Heart of Gold. This item is what lets you get fed. The health is nice but it lets you get to your immortal build so much faster if bought early. You will be tempted to get Boots first (after dorans) but don't get it right after. Then get your kindlegem. More health and CDR which goes right into Spirit Visage. You then finish your Spirit Visage and Mercury's Treads and thus have a solid core build for early game.

MID GAME:
This is where your enemies will really start hating you. Next item is Sunfire Aegis because your armor should be pretty weak for a tank now. Decide based on the other teams physical damage output whether the Chain Vest is needed or just troll them harder with more health in the Giant's Belt. Some Mundo's rush Warmog's Armor but you need to have the resistances and gold gathering ability higher first. Once you have the Sunfire Aegis and Warmog's Armor directly after it just turn on Heart Zapper at a large creep wave and watch the Mog's stack real quick. That finishes my balanced build for Mundo.

LATE GAME:
You now need to decide whats important for your Mundo. At the top I posted the 3 Warmog's Armor High HP troll build, which is actually pretty balanced and good. I have sections for more specialized builds.

This item is actually really good for Mundo but I prefer Mercury's Treads for the CC reduction and MR. But more cleavers and ults is also nice.
Almost too much troll on this one.

Ninja Tabi
Spirit Visage
Thornmail
Sunfire Aegis
Randuin's Omen
Warmog's Armor
Never go all AD agasint a mundo on TT EVERRRR. This is why.

I have literally made a lategame Tryndamere attack me, realize he takes more damage then he deals, and then run away as he pops his ult. Trolololol.

If they do get a Madred's Bloodrazor just buy a Force of Nature.

Mercury's Treads
Spirit Visage
Warmog's Armor
Frozen Mallet
Force of Nature
Another Warmog's Armor?

I generally don't get scared of magic damage dealers as most are slow casters. So I get a Frozen Mallet to slow them down even more. This build is very rarely used if ever. Most people on 3s try to go AD.

Infected Bonesaw may be one of the best skill shots in the entire game. I has a flat slow, %health damage, low CD and fast particle speed.

Now all I can say about using Mundo's Cleaver well is practice. I am decent, no pro, but I get the cleavers that secure the kill for my team. I do have some tips I have found:

1)Don't be afraid to use the cleaver. Worst case, you miss. Best case, you hit an enemy champ and now they consider backing off.

IN A CHASE
2)Hold off on the cleaver if they're trying to juke. I know you want to throw, throw, throw. But landing a late cleaver is better then missing 3 and they get away.

3)Best time to land a cleaver in a chase is when they have to make a turn or through walls. I have nabbed kills, saved fleeing teammates, and got that slow to hold back a fleeing enemy countless times through a wall. Most opponents don't see the cleaver coming through a wall.
3a)When the fleeing enemy has to make a U turn or just a normal turn(many places on TT) it is the best time to get that slow for an enemy almost out of reach. And always remember RULE 1. Always throw the cleaver. Who cares if you miss, that one time you kill the stealthed teemo with one sec left to back makes it worth all the misses.

A good Mundo from a bad Mundo can be separated by their ability to land cleavers and their presence on when to ult. I generally ult on several scenarios:

1) Because of Spirit Visage's CDR I dont mind just using my ult to save me from recalling so I can sit in lane and farm XP even if I can't farm minions because my ult down. (This ability to lane stay has caused me to reach level 18 while opponents who have been zoned and forced to recall are still at level 11... Sometimes causing problems when I have so much gold I buy Warmog's Armor flat out haha.)

2)At the beginning of being ganked with no help around. Pop it, who cares. Gtfo and regroup, your ult will be back soon.

3)During a committed teamfight. Normally I run into the think of the fight and when I get low use heal, then I pop my ult. This initiation causes me to get focused while my DPS score at least one kill. At times they may kill me, but most times my DPS's can manage to kills back in return if not an ACE. Worth. (You also get focused better in a fight if you piss them off in the early lane phase =P)

4)SOLOING DRAGON. With your level 4 Infected Bonesaw and level 1 ult, you should be able to take down dragon while you keep Heart Zapper up. I will normally solo around levels 7-9 after I back. Noone expects the main tank to go solo dragon at such a low level while the teammates are in lane (key step! make sure you solo or you'll have a teamfight on your hands) I solo dragon about 4 times per full length game. It frees up farming for your other two teammates in lane.

5) Using your ult as a speed boost. Rarily used. Only used to get that kill your team needs badly or if you feel so tanky you don't even mind "wasting" your ult.

LEVEL 1:
//GANK\\You should have started with Doran's Shield and Infected Bonesaw. When you go for bot first blood charge because of you speed buffs from runes and masteries. Then just target the stupidest player (not squishiest) the one that commits most. Tag them with a cleaver and exhaust them and watch your team go to work. You then pop heal because you committed hard and now you have first blood.

//MINIONS HAVE SPAWNED\\ If you didn't get FB oh well. If you got aced, that sucks but your Mundo and will rebound. Now your role bot lane is to cover your DPS. Take skillshots for them, aggro minions, and piss off the enemy with your Infected Bonesaw. The combination of Doran's Shield, Vigor , and the early health bonus coupled with you passive Goes Where He Pleases will allow you to quickly regain your health if you sit back and take no damage.

//LEVEL 3\\ So you have a Level 2 Cleaver and Level 1 AOE. You job now is to pop your AOE in minions waves to melt them as you gain gold by lasting hitting either with your basic attacks or with the cleaver (extra last hit attack). Bladed Armor Indomitable and Tough Skin allow you to tank all minions in a wave with ease considering they do like 20 dmg each anyway. Once enemy minion waves are cleared their is no shield for cleavers and hack away at you opponents.
-If they have no ranged harass ignore the above and just zone them. :)
At the end of this you should have somewhere around 800 gold hopefully. Go back and buy your Heart of Gold to feed you as the game goes on.

//The Return of the Mundo\\ Come back to you lane and do the same thing and follow the course of the game. I cannot tell you what to do here because obviously each game is unique. But I do have some goals I have addressed earlier:

1)Gank top lane when you can. Mundo is a great pusher bot lane, so if top is pushed back and your pushed foward, lend you top lane some help even without the kill, just give them the ability to fight back if they're beat back.

2)SOLO DRAGON. Once you have your ult and preferably a tower down, tell your teammates to take each lane and farm carefully. You go solo dragon with Cleaver and AOE popping you ult about a third of the way just in case you get ganked while at dragon (i.e. pantheon, TF, nocturne,karth)

3)Even if your teammates are switching lanes and jungling or whatever DPS's do, just STAY IN YOUR LANE and farm gold and xp. Help them obviously when necessary, but don't go chasing Singed around the jungle while Caitlin gets her 40th minion kill at level 3 :P (Two good examples, baiting troll and minion farmer)

Simple Rules.
1 AD Champ
1 AP Champ
Mundo.

NO OTHER TANKS. On TT if you go more then one tank you fail. Proven.
Double AD works because Mundo does mostly magic, but not triple magic damage.

My friend and I normally double queue and play with a random and hope its an AD. If not I play someone else. My friend normally plays Brand or Malzahar two nuke casters who required a big brother. (not as much malz)

Went back to Mundo for this guide and did quite well...

Be a critic if you want. I will try to cater to the public when I feel like it. But I did this guide for fun. I find success in my Mundoing and would like to share with other upstart Mundos on the Twisted Treeline. GL HF