Hello, you probably won't even read this so lets just get started.

Pros:
Very durable
Good slowing ability
Good initiator
Fun and easy to play

Cons:
Weak aginst % of health based damage abilitys
Ignite is not your friend
Minions can get in the way of a kill

Base health is always nice when making a off-tankish bruiser, armor and magic resist is great to keep the damage coming to you a minimal.
You could also try going health regen runes, these are quite useful as well.

Defensive tree is great for mundo seeing as his main nuke is a skill sot based of % of health which cannot be amplified by armor pen or physical attack boost. I also go into the support tree to grab the movement speed boost, mundo is very fast and can catch up to almost every champion with ease.

I build mundo like a tank, he has great movement and shrugs off most damage any team can put out to a degree. I grab boots of swiftness mainly because of Initiator and Swiftness, mundo is amazingly fast and it's very hard to out run him with these on. I tend to make Force of nature first, this with his passive make him very durable while laning top, he also has no mana which means he can stay in lane almost definitly. Spirit visage is a no brainer it increases his passive regeneration along with Force of nature. Guardians angel is amazing, most champions grab this simply because if they do for some reason focus and kill you, you can revive and ultimate if you have not used it yet, making it seem like you have 3 lives :P. After those 3 items are bought you really can just go by what you think you need most, I tend to make warmogs and then atmas simply because, more health never hurts, and more damage is always nice :).

Mundo's skills are pretty straight forward, your Infected cleaver does massive damage (unless you cannot aim), and burning agony allows you to get out of crowd control effects faster along with doing area of effect damage which is awesome for team fights. Also burning agony is a very useful chasing tool, with boots of swiftness and your mastery talents you can always catch up for the quick and definite kill. Your masochism is useful dont get me wrong but I believe the other abilitys are more important, although this on is quite nice when taking a turret down or creep farming. Your ultimate like all champions should be maxed as soon as possible, Sadism is my favorite ultimite in the game, it lets you heal almost completely, it gets the Spirit visage effect, and is great while team fighting.

Exhaust: simply great for helping gank, or helping your team finish off that pesky AD carry(Teemo -.-).

Teleport: I love this spell. Mundo hardly ever gets forced back to have to heal because when he is low he can just hug the tower and have full health within a short time. Grabbing this allows you to go back every 5 mins or so if needed to grab your items and quickyl get back to your lane without having it be pushed to hard.

All in all I know this probably isnt the best build but whenever I use it i find myself almost indestructable and my team mates love my skill shot slows, and that i simply cannot be outran by almost anyone (I think yi is the only one to get away so far). So if you want to "almost never die" then follow this build and I ensure you will be a great mundo :D!

"Mundo too strong for you!" - Dr. Mundo the madman of Zaun

Thanks, Nero