Since I'm reading Harry Potter, I'll admit I've given this some thought.
- Lilah is a Slytherin obviously. She's ruthless, ambitious, resourceful.
- Darla is Slytherin too. She wants power and likes to manipulate situations and people to her advantage.
- Faith is hard for me to place, but I'd say she's more of a Grynffindor. She's strong, brave and daring. However, I always saw loyalty as one of Faith's best qualities. Faith was loyal to Buffy in the beginning and then became extremely loyal to Angel. She's not particularly patient or a fan of fair play though, so she can't be Hufflepuff.
- Wesley is a Slytherin I think. He uses knowledge as a tool. He doesn't want knowledge for the sake of being knowledgeable but rather as a resource. Wesley is above all pragmatic and ruthless in order to achieve results.
- Angel is a Gryffindor, I think. There's a lot of Hufflepuff in him as well. He's loyal, hard working, patient and quite fair. But, mostly, as a champion and a leader, he is daring (he dares to break the status quo and do the unthinkable), he is brave (faces death on a daily basis) and he has nerve (enough never to face forces more powerful than him and to defy them).
- Connor is a Gryffindor too. He is just like his father in terms of being daring, brave and having nerve. He killed Jasmine, faced beasts and demons in Quor'toth, fought The Beast, fought Sahjhan when he didn't even remember his fighting skills, fought Hamilton for his father, etc. He's also chivalrous.
- Gunn is a cross between Hufflepuff and Gryffindor to me. He's all about hard work, dedication, loyalty and fair play. He also has a lot of neve and is daring and brave. Considering his role in Angel - as someone who follows Angel but doesn't lead - he's more of a Hufflepuff.
- Lorne doesn't seem to fit in any of the houses? Maybe he's more of a Ravenclaw because he's so authentic? He's also full of knowledge, especially as an empath demon.
- Holland Manners and Hamilton are Slytherin.
- Fred is a cross between Ravenclaw and Slytherin. She loves knowledge for the sake of knowledge itself and is quite quirky. I'd say she's a Ravenclaw.
- Cordelia is a Gryffindor. She's not ambitious enough to be a Slytherin.
- Xander is a Hufflepuff (although he's plenty brave, his dedication, hard work and loyalty are bigger), Buffy is a Gryffindor, Giles is a Slytherin, Willow is a Slytherin.
- Dawn could be a Gryffindor, a Ravenclaw or even a Hufflepuff. In season 7, she's more of a Ravenclaw, but I think she's more of a Gryffindor (as every character apparently).
- Tara is a Ravenclaw, I think. She's authentic and different, and a witch who seeks to always know more.