I suppose there are different interpretations of what being a better Champion entails, but any way you slice it, I think the answer is Buffy.
In my opinion, the main thing that makes a Champion more heroic is his or her personality. And, contrary to the popular belief that her heroic attitude was thrust upon her, I definitely think that Buffy chose to be a hero. All being the Slayer really did was give her superpowers, and she chose to use them to help people, even though she could have been like Faith and abused them just as easily. Angel, on the other hand, had fairly selfish goals...in a way, all he was trying to do was boost his own self-esteem. That leads me to another big thing...incentive. Angel was always working toward something, whether it was redemption or the Shanshu. All Buffy was working toward was an untimely death, and, even though she knew that, it didn't stop her or even really faze her.
Still, some people could argue that being a better Champion means saving more people. I suppose Buffy sort of had a leg up, since she was physically stronger than Angel (not by a lot, but still) and, before Angel season 5, her team was way stronger than his, but, even when Angel had the most powerful organization in the world backing him up, he wasn't able to do as much good as the Scoobies. I mean, think about it. Buffy saved the world and possibly the whole dimension at least eight times and stopped what was arguably the strongest evil in the universe. Angel saved LA maybe three times, and wasn't even able to bring down Wolfram & Hart when he was running a branch. There's really no comparison.
