I think I wanna say Spike (not just because its Spike) because to me Angel seemed to wanna look after Buffy more so was never fighting at his best. Where as Spike knew that Buffy was strong and could look after herself so wasn't worried and distracted by that in a fight.

em_em said it all. another reason why spuffy is the best :)

I want to preface all of this by saying that I'm currently playing link in the background while I write this.

That said, I personally believe, as em said, Spike was stronger with Buffy. Angel, I believe, felt that Buffy needed to be protected while Spike loved her strength - it turned him on actually. Angel will leave Buffy because he feels that she is too weak to make her own choices while Spike is always willing to do whatever she asks of him. Spike trusts her and treats Buffy as an equal - their relationship is completely give and take.

In Earshot Buffy's biggest desire is to hear what Angel is thinking and she still can't do that and Angel, though he tells her just to ask him, isn't that open with it all. Spike on the other hand bares his heart and later on his soul, gives them to Buffy and let's her do her will with them. The strongest character isn't the one that stands stoic but the one that allows you to move his foundation. It's braver, in my view, to risk it all than run from possibilities.