If the Tsubasa gang appeared in the Harry Potter world and studied in Hogwarts, these would be their houses to me:
(And Yuuko the headmistress)
*EDIT* People seem to be asking about my choices a lot, so here's an explanation (it may contain SPOILERS!)
Syaoran: He's brave and his motivation in life seems, above everything else, to protect Sakura.
Sakura: She really brave too, just... read the Tokyo arc. And after the Tokyo arc. And everything, she's just amazing like that.
Kurogane: His loyalty is what stands out the most to me. He's loyal to Tomoyo, and he works hard to become stronger and protect those he loves. Loyalty + hard work = Hufflepuff.
Fay: To me Fay was both Ravenclaw and Slytherin. Why did I choose Slytherin? Because he's cunning and has a not so healthy ambition in life. At first he agreed to do whatever he was told (even to kill someone) in order to fulfill his wish. Being Slytherin doesn't mean he's EVIL.
Mokona: I think it's obvious, loyalty and hard work again.