You should always build according to your enemy team, meaning if the majority of them are physical damage build a thornmail before a force of nature and likewise if there is more ap on the enemy team build force of nature first. Starting out with a ruby crystal gives you an extra 180 health which is great early game, building that eventually into a warmogs. After you buy your first warmogs i recommend buying some armor and magic resist. I would then build a second warmogs or frozen mallet according to how you do during the game. Then You should build a thornmail or buy an atmas if you are getting fed and end the build with force of nature. Mundo is great on 3v3 and when you have mastered his q skill shot you can be a great mechanism towards slowing your enemy team. Mundo's w is great in team fights as it does burst damage and his e gives you an extra attack damage boost. His ulti gives him a movement speed boost and heals you overtime. I would not hesitate using mundo's ult as the cooldown is pretty quick.
Mundo can be played solo top, mid and jungle and so can be played in many ways.
I would max q first get ult 6,11,16 and then either get w for mroe aoe damage or e for more 1v1 counters

Pros:- If built correctly can be a great tank and unstoppable late game. His q is a great source of harrass and farm. His r will give him great sustain and let him stay and lane.
Cons:- Since his skills cost health ignite can easily counter mundo also the item executioners calling can be counter mundo.