This is the most effective Mundo build for me i tried out some other builds but they didn't worked out for me so here u have my personal best Mundo build!

Play Mundo offensive when ever its possible use Infected Cleaver and Masochism and when your hp is to low use Sadism(Mundos ultimate)!

I take Armor Penetration Marks,
Health Seals,
Health per lvl Glyphs and
Health Quintessences!

This is the best choice for me Because Mundo needs health and health regeneration because his attacks uses health!

I take defense masterys because Mundo is a tank he can kill other champions but the way to play Mundo is to have defense masterys!

Take Mercury's Threads. I take Spirit Visage and 2 Warmogs Because this is the best for Mundo! U can take Force of Nature between the Warmogs or just like me one after the another. And the last item Randuin's Omen for some health and and armor!

For the starting items take Ruby Cristal or Regrowth Pendant!

First of all u should open Infected Cleaver because its the most powerful attack that Mundo has.
Then open Masochism because your attack damage is higher!
If u want to farm minions then open Burning Agony but else try to max. Infected Cleaver and Masochism.
And don't forget to open Sadism when possible!

I use Ghost and Ingite! The most effective Summoner spells for Mundo!
When more then 2 People chase u just use Sadism and ghost and they will never get you!

Pros-
High hp and hp regeneration
Strong attacks
Quick (fast)
Strong Survivability (if not ignited)

Cons-
Only one affective attack
If Ignited low chance to stay alive

Try to slow the opponents with Infected Cleaver and use Sadism to stay alive as long as possible!

When u decide to farm with Mundo then max. up Burning Agony and u can farm minions all the time! But i don't recommend that because its only wasting of skill points!

At the end i can say only one think i hope u liked my build and enjoy playing Mundo!