This question is two-fold - if one was to go simply off the book - the canon so say you - the obvious answer is Gryffindor. JK was not even remotely subtle in her favoritism. Obviously she feels courage is the answer to overcoming evil. On the other hand, if by just looking at the sorting hats and definitions of the four houses, then the word favorite would need to be defined. Favorite for what? Frolic? Achievement? Character?
All the houses, outside the book, (in the book Huffs are totally under represented) have gifts and contributions. It was no coincidence that at the end of the series, all the houses were mixed.
Long way of saying - favorite house? Each house. Depends on what I need.