Dr. Mundo in my opinion is the best tank - with this health and health regen, he can't die ! He is one of my main champions too. Let's start this.

Pros :
Nice health regen.
Great farmer.
Really usefull abilites.

Cons :
Focuses a lot early game and if you can't play iwth this champion, you won't have good late game.

Absolutely defencive (tank) runes.

Spirit Visage - Nice combination with your ultimate !
Ninja Tabi - More dodje chance. (If enemy team got much casters, purchase Mercury Treads)
Warmog's Armor - Bunch of health.
Force of Nature - High boost of health regen and magic resist !
Atma's Impaller - Well, you have 4k+ health and this item gives you damage, armor and crit !
Hextech Gunblade - You will say 'A hibryd item on tank ?! WTF' and here is the answer - You have verry much health, health regen, armor and dodje but you don't have verry much damage. This item gives a lifesteal and spell vamp, nice unique ability - you are going to be LEGENDARY !

Ignite - Nice DoT ability that will make kill secure.
Exhaust - Verry usefull at all team fights.

Another options :
Heal - Saving you at early game. But not too much at late game.
Flash - Nice escaping ability.

Thanks for all that have red this guide. I really sorry for bad english but it's not my language. Test the build before posting a comment, please. (The guide will be updated !)
Greetings, Bladestorm xD.