It's a sort of potato...

If I had to pick the most brazilian starchy I would answer without hesitation: "Mandioca!"

Manioc or Cassava in English
Yuca in Spanish.
Macaxeira at the north of Brazil.

I took myself once giving an explanation about manioc to some german friends: "It is kind of potato.." It can be wrong from the bothanical point of view, but It made sense at the sentimental aspects to germans.

Cassava was considered some food for poor people and it was banned from the menus in "fancy/snob" places. Now, brazilians are starting to be proud of being brazilians and serving local dishes with manioc.
Like this fried "mandioca", It was really tasty...