Hey guys, this is my first guide, so any feedback is appreciated and I apologize for any inconveniences or mistakes. Also, 5forwards did a good job on his Dr. Mundo build, and a lot of my build is similar. His build is here. Also, if someone could tell me how to link in items/abilities that would be awesome. :P

Dr. Mundo plays as a very HP-heavy tank, with large amounts of health regeneration and movement speed. He also has a very powerful DPS output, which is part of what makes him very fun to play.

Pros:

Very tanky with his HP and regen
Can stay in a lane almost indefinitely with his ultimate
High damage output for a tank
High movement speed and maneuverability
Low cooldowns on all of his abilities
Not very item dependent at all

Cons:

Kind of weak early game, not as tanky
Almost squishy until you get your ultimate
Somewhat dependent on cleaver harass, missing several times in a row hurts your regen
Cleaver doesn't scale with anything
Not as much armor or magic resist as other tanks

Honestly I haven't experimented much at all with runes, but HP helps a lot early game, which will help you survive until level 6, and any tank runes will help. (armor, magic resist, health, dodge) 5forwards does a better job with the runes, so check out his build if you want more of a cookie cutter rune build.

Mundo has kind of an odd mastery build, going 8/21/1

I go in offense and pick up crit and cooldown reduction, with a standard defense mastery except for when I pick up Willpower, and finally I grab Haste in utility.

With the cooldown reduction from masteries and his cleaver gets down to 3.4 seconds I believe at rank 5, so 25% of the target's current health every 3 seconds is pretty awesome, as long as you can land the cleavers.

Items for tanks greatly depend on the enemy team. The number of casters and auto attackers on the enemy team affect your build.

For a Balanced AD/AP Team:

First I grab a Regrowth Pendant and a Health Potion, though sometimes I'll grab a Ruby Crystal if I'm laning with a friend and know we'll want to try for first blood.

Next I grab Boots of Swiftness for the movement speed, which brings you up to pretty high movespeed with your ultimate and ghost. However, if their team is high on CC,, you'll want to go with a pair of Mercury's Treads.

After that you want to try to build up your Spirit Visage as fast as possible. This is pretty much a mandatory item for Mundo in my opinion, as it gives magic resist, health, cooldown reduction, and of course a boost to your already insane regen from your ult. :)

Next up is Warmog's Armor, pretty obvious there. Lots of health, Lots of regen, lots of regen from your passive because of the lots of health. Also, once you get this try to farm as much minions as you can to build up and max the health bonus as soon as you can.

However, choosing to get Spirit Visage before Warmog's Armor is my personal preference, or even getting Boots of Swiftness before either of them. If you feel that you're too squishy and could use some hp instead of speed or magic resist/cooldown reduction you could definitely choose to get those three in any order you like, it's not going to make or break your game either way in my opinion.

I used to go Thornmail next, but I've tried Randuin's Omen and thought it superior by far. Health, regen, armor, slows attack speed on hit, and has an active that slows the attack and movement speed of every enemy around you. As you are tanking and going to be initiating, this is a great item to pop right as you get in the middle of the enemy team.

After that I'll grab a Force of Nature next. It has a 0.35% of your max health as regen that stacks with your passive quite nicely, plenty of flat regen, magic resist to stop those pesky casters from nuking you, and even some more movement speed to initiate, retreat, and chase faster.

Lastly I'll grab a Guardian Angel for the balanced armor and magic resistance, and obviously the passive comes in handy.

For a Heavy AD Team:

Follow the same build as above, except grab a Thornmail or Sunfire Cape or even an Atma's Impaler instead of a Force of Nature.

The choice between the Thornmail or Sunfire Cape is based on how badly you need armor. If the physical damage is just murdering you, grab the Thornmail, if you aren't having much trouble, you may want to grab the Sunfire Cape for the added health and passive that stacks well with your Burning Agony. The Atma's Impaler is for if you are facerolling the other team and just want some extra DPS.

For a Heavy AP Team:

Again, follow the first build, except grab Mercury's Treads instead of Boots of Swiftness and maybe a Quicksilver Sash instead of Randuin's Omen, but after you grab the Force of Nature.

The CC reduction from Mercury's Treads stacks nicely with your Burning Agony, and the Quicksilver Sash is an extremely useful item. It has a nice bit of magic resistance and a great active.

REMEMBER TO USE THE ACTIVE! This item can remove ALL DEBUFFS, including the healing and regen reducing ones like Ignite, Katarina's blade, and Gangplank's poison. These debuffs can greatly hinder your regen on your ultimate if you use it while they're on.

A fellow Mundo'er Adonikam stated in one of his old builds, that the key to Mundo is his MOBILITY. As shown in this diagram below from his old build, you need to keep moving, even while laning.

Early Game:

Always be looking for an opening for your cleaver, as it will hit minions and can be very annoying that way. While the diagram shows what to do as middle, you will want to be going top or bot for this guide.

Be moving around, trying to get a spot where you have a clear shot to the enemy champions. Go into the brush often, as they have a harder time blocking cleavers if they can't see you.

One way to check if your cleaver will hit your target is by looking at the thin blue lines on either side of the arrow when you click your cleaver. Those two thin blue lines show the area that the cleaver will hit, so if one of those lines is overlapping your target, they will be hit. This also means that if an enemy minion is overlapping a line closer down the arrow to you than your target is, the cleaver will hit the minion.

If you can land a few cleavers on the enemy champion they will get fairly low and possibly have to go back. Sometimes if they're low enough you can catch them trying to teleport back and be able to run in and throw your cleaver for the kill, just be careful as you can be fairly squishy until level 6.

Also, because of your squishiness, you should always be using your cleaver to check the bushes.

Mid Game:

Your mid game starts when you get your ultimate at level 6 and start getting a lot tankier. Try to keep pushing your lane and get a few towers down. Whenever you attack a tower be sure to keep your Masochism up. However, be ready to use your ultimate and ghost out should the enemy team try to gank you.

If you are getting ganked, your speed from your boots, ult, and ghost should be enough to get you out of there. If not, remember to keep your Burning Agony up for the CC reduction, but take it off if you get low so you don't get yourself killed. Also remember you have cleanse, so be sure to cleanse out of stuns/roots/slows as needed.

Late Game

Your late game begins once you get Randuin's Omen. At this point you will be getting into team fights and will need to start doing some serious tanking. Remember that you will be initiating, and try to take as much of the fire as possible.

Before the team fight, you will want to try to harass the enemy team with your cleaver to get their squishies low. You'd be surprised as to how much damage a rank 5 cleaver does to a Teemo or Ashe. Once you land a few hits with your cleavers, it's time to initiate.

Be sure to use your ultimate BEFORE you run into the other team. If you wait too long the 20% health cost could kill you, and you won't regen as much. However, don't use it way too early or the effects will wear off before you take any serious damage.

After you activate your ultimate, run into the middle of their team and activate your Burning Agony. This will help deal some AoE damage and reduce any stuns or roots on you during the fight.

Once you reach the enemy team, use your Masochism, and make sure to keep it up throughout the fight. Of course, you should also be using your cleaver as much as you can to deal damage to their squishies and do some major damage to armor-and-health-stacking tanks.

At this point you should also be able to tank towers, so use this to your advantage. Backdoor turrets, tank them for your teammates, and initiate tower dives to help with pushes. But all the while make sure you aren't missing team fights, as your team will need you to be a damage sponge.

Thanks to Adonikam, whose guide helped me start with Mundo and thanks to 5forwards as well. Hope you two didn't mind me referencing your guides, which are both very good guides. :D

Again, this is my very first guide, so leave me some feedback, let me know anything I did wrong or anything like that, and also I would really like to know how to link in items/abilities to my guide, would help a lot. :P