Depend on the reason: to break traditional view of the houses, either Hufflepuff or Slytherin. Would see a lot of DnD fans from Hufflepuff, as well as avoid the sterotypical "slytherin==evil" and avoid being in Snape and Malfoy's house. Of course, I feel the Hogwarts Houses have been corrupted like the Ministry. I like to see the Hufflepuff mascot as a Hellhound, Slytherin as a Dragon, Ravenclaw as a phoenix, and Gryffindore as an actual gryffin.
Ravenclaw would work too.
But Hufflepuff would be more interesting.