Favorite Slytherin. I've had someone in mind for this one for a while too.
The introduction of Horace Slughorn was a really interesting point in the books for me. He is the first character that Harry and I disagreed on, and therefore marked the first time I realized that these books are written from a limited third person POV, and Harry's not necessarily right about everything.
I quite like Slughorn, and I don't really blame him for the way he goes through life. So he likes nice things, so what? He's willing, even though he's a Slytherin, to give 'blood traitors' and non pure bloods just as much of his respect as pure bloods.
He's just a cute, chubby old man, and I find him both entertaining and an extremely interesting character. He was also the first character to challenge "all Slytherins are evil," which I think was a deliberate attempt by JKR to make her world less black and white.
So Slughorn is my favorite Slytherin both because I like him as a character, and for how he elevated the books to a new level.
And because he helped my bb Ron. <3