A tank is needed in nearly all games, and Dr Mundo is in my opinion the best! Dr mundo deals tons of damage mid/late game and Becomes allmost unkillable! He will not just be a tank in teamfights mundo can deal alot of damage to single targets and has an amazing slow. Hes Q is a skillshot that deals damage to a percent of there max health so deadly all stages of the game it also has an amazing cooldown with an amazing slow. His W spell causes a ring of fire to circle mundo dealing damage, Also it reduces the time of cc on mundo, mundos W will be up 24/7 on mundo late game. Dr mundo's E is a buff spell that increases damage done by basic attacks by a flat ammount plus an increasing ammount the less health he has, this makes him do an amazing ammount of damage with his basic attacks! Dr mundo Ultimate is a buff spell that make mundo unkillable, he trades a % of his current health for a health regenertaion increase on his max health in my opinion the best ult in the game.
http://screensnapr.com/v/3FTkQ1.jpg
Some proof that i this build owns.

With mundo you need to rush a warmogs armor to give you your health and regen. with all that health armor and mr fits in perfectly and you can become very tanky early game. The destruction marks make sure you still do tons of damage with your basic attacks and your spells. The health quintessence's Will help you get the health you need to become unkillable.

warmogs for health FoN for regeneration and mr and late game atmas for damage on basic attacks.

ghost to be a fast unkillable machine and teleport to ninja around the map and help out lanes.

Spam Q to make sure there not getting away Use W and E to cause damage, if your gona die pop your ult and get the hell out of there or kill him.

Pros to using mundo
-Unkillable when full build
-amazing spells to deal alot of damage without damage items
-Great passive helps alot all stages of the game
-Ultimate best in the game
-The best tank for staying alive and dealing damage
Cons
-if aimed without items can die fast
-Alot of gold for full build
-Doesnt deal as much damage as some other tanks
-hardly any of his spells can deal more damage with items.
-Needs a strong team to back him up
-can get ignored in a teamfight

Thanks for reading my first guide on mundo, Gl hf and own bad kids, and if you didnt own with this build you suck **** ****** ****** who has aids and a big forehead (like preece who begs for ganks cuz hes ****)