Well, where on earth is the evidence that (say) Jesus was a better speaker than (say) Cicero? I'd say there's evidence that Jesus and Muhammad, at least, were decent speakers; they certainly had a dramatic effect upon their audiences. But I don't see any evidence for saying that Confucius and Buddha were. In fact I don't think that either of them was noted for giving speeches to large audiences at all. Confucius, after all, wasn't very successful as a politician, which doesn't seem good evidence for his having been a notably impressive speaker.
The Christian preachers generally considered the best speakers include Gregory Nazianzus, John Chrysostom, Augustine, John Wesley, and (perhaps above all) George Whitefield, who was said to have driven fifteen people mad with one of his early sermons. Savonarola should probably make the list too.