Actually this question is not fair to me as I have not visited all the regions of Brasil.
The people make a city or region attractive and to date the vast majority of the people of Brasil that I have got to know are honest, hard working, friendly and fun loving. While each neighborhood will presumptively have both good and bad inhabitants, my experience is that the vast majority of the people of Brasil are honest, hard working, friendly and vavacious.
From a strictly natural beauty point of view I would have to say that my favorite places are, in no particular order, Paraty, Fox do Iguacu, and the Pantanal.
I absolutely love the contrasts in architecture that one finds in almost any city, the vibrant colors, styles and age of the buildings are absolutely indescribable, and must be seen to be appreciated. Of special beauty are, again in no particular order, Luz Train Station, Metropolitan Cathederal, and Municipal Theater in Sao Paulo, Amazon Theater in Manaus, all of the Government Buildings in Brasilia, Christ the Redeemer in Rio, and almost any church in any town.
So in answer to what is my favorite city or region in Brasil, I must say that all of Brasil is my favorite. Ask an unfair question and get an evasive answer.