For all Buffy's creative thinking in battle tactics, I can't help but think she has a rigidity of thought when it comes to judging people. Angel will always be the mysterious hero to her because that's who he was when she first met him.

Spike enters a villain and it takes forever for her to change her mind on who he really is.

With Angel, I don't think she ever really changed how she views him... maybe in Season 8.