And the First. But really they barely stopped Willow from destroying the world and Buffy wasn't the one to stop her. If you think about it, if Willow hadn't been their friend, if she had had no emotional connection to them there really wouldn't have been anything they could do to stop her.

I agree, Dark Willow was rather unstoppable but don't u also think that similarly Buffy had so much trouble neutralising her BCOS of their emotional connection, the fact that they were friends could've been the reason y Buffy held back somewhat in stopping her?

^True but that just made Willow even more dangerous to Buffy personally because of Buffy's hesitance to hurt her friend. But there was a point where Buffy stopped holding back and she was still unable to stop Willow. Either way you look at it, Willow was an extremely dangerous foe.

Glory was a Goddess and was so hard to fight. The only way to kill her was in Ben's form, and Giles took on that burden because he didn't want Buffy killing an "innocent". Also, Glory's actions led to Buffy having to sacrifice herself.

I'd say that Angelus was a close second simply because it was so personal and the guilt of having unknowingly released him.