Oh, I kind of love all of her hairstyles truth be told... well maybe not the short bangs she sported for a while in season 3, not that she wasn't adorable in them, she was. I'm just not a fan of that kind of short look.
My first thought though was season 2, cause I love that simple short do, it's so... Buffy. All of her later longer do's are gorgeous as well though but I don't know, season 2 was especially cute I think.