The largest human organ is our skin, it is airtight, waterproof, flexible and fallible.

Weird as' question:/ cos I said so. 

It's neither airtight nor watertight - we perspire.
Personally I think an exoskeleton woukd be pretty cool.

A tortoise shell would be handy at the beach. I've never heard of a sunburned tortoise.
Then there's a rhino's skin. Useful for public servants who routinely get back-stabbed at work.
Goldfish skin would keep you dry in the rain. Well, sort of. 
Bear skin would be great in the winter but might be a bit too warm for the summer.
And porcupine skin would be magic to help you push through the crowds to get a front row view at a parade.
If you ask me human skin is overrated. 

This post was edited by Benedict Arnold at March 15, 2017 6:31 AM MDT

At times I am sensitive, during those times I wish I was this;))

Skin is not air tight or water proof.   The skin as an organ constantly absorbs things from the atmosphere and exudes/expresses on a consistent basis.    Its in fact a quite breathable physical covering.    Flexible, yes.   And im glad you said 'fallible' rather than 'infallible' because, as awesome as skin is as an organ, it is quite fallible.  Fairly easily damaged despite its other protective qualities.    Still the flexibility and breathability alone i think make it ideal.  Cant think of any other animal covering i would want.   Most others would be too inflexible or cumbersome for human needs.  

This one...I could make my own food.