Telegraph readers are clearly a bunch of historical illiterates, since Alfred the Great didn't even get a single percent of this vote for greatest British monarch. Has Brenda ever defeated a single Dane, I ask you? It's right there in his name! She wouldn't even be a good card in the Top Trumps set (ratings: Weight, Height, Wives Executed, Nobles Imprisoned, Foreigners Killed, Parliaments Dissolved, and Miles Per Hour.)
Globally, I think King Sejong would probably top all the polls, not only on merit but because Koreans can swamp online surveys like nobody's business. Partially because of the widespread literacy and love of technological innovation begun by King Sejong, so there you go.