"Who is your favourite 'character' in Buffy and why?" as well as;
"Who is your favourite 'character' in Angel and why?"
My favourite character in Buffy would probably have been Spike. Reason being that he was a great villan. He has spent more time actually trying to get the slayer than most, and not by any super special means, but by the method of actually beating her to death, which is oddly refreshing.
He never did, but he was always trying, which is also refreshing. He also had a great history with Drusilla, and Angel, even Darla, and a bit with Harmony too which makes him an interesting charcater. He actually makes a connection with almost every character in some way. And even with the chip in his head, he still likes to belive he is evil, and beats up anyone he comes across, but its always seems to be for a good reason, though he would hate to admit it, he knows that too...
For Angel, I think it would be Cordelia. I also really like the Host, but Cordy has this thing about her. She came to make it as a movie star, and ends up working for a Vampire with a soul in a detective agency. She becomes such great friends with someone who she would have dissmissed back in Sunnydale, and she has grown up much faster than the others in Buffy, which is really good!! She lives in the middle of LA, each and every week she has to help track and kill some demon, or a hoard of them, and she also has to deal with the power of the visions... I could go further but without risk of spoiling something I won't.