It seems that the poll is missing too many famous orators really to be considered a "greatest of all time" competition. A few who deserve mentions but don't get them include Lloyd George and Joe Chamberlain, who apparently were reckoned in their day to be far better than Churchill. And indeed Hitler was clearly an inspiring speaker, and ought to be included in any list of great orators. But the thing is that, Churchill aside, and some snippets of the others, I haven't listened to all that many speeches, not in full anyway. Blair's speech on leaving Labour is one of the few modern ones I've heard in full, and I don't know what I think of it really.
We don't rely quite so much on long speeches any more, so it's likely that the art is relatively poor these days, not because people are necessarily any worse at public speaking but because they don't have to be. For good or ill, we're in the age of the soundbite.