best tank?

You guys are talking about being the best tank. The best tank is suppose to be the one who cna mitigate most damage, Mundo is good, but his strongest point is not mitigate damage, its his overhealing from his ult, having an ignite or an item with griveous wounds would cut his healing leaving him wide open to damage and shut downs.
Malphite isn't a bad pick but, if you really want the best tank, only one can surfice that title, and thats my boy Rammus, believe me, that boy can not only tank for days, he can also kill enemies without even attacking them, his whole kit is made for him to be the tankiest of the tanks and to punish those who try to defy his title.
Go ahead and try it by yourselves, you will be running at the enemy team 1v5 and leave from the battlefield alive with 2 or 3 kills, in a few cases even a pentakill. I can't think of any champion who can do that by himself other than him.