Arguably the most famous of all contemporary opera singers. Actually, that's not really debatable. While I think not all he did for opera was necessarily beneficial to the genre, he certainly brought opera to the mainstream public.
Actually, it's NOT Pat Boone, because of some copywright issues MTV couldn't get around (Pat Boone recorded a CD of metal covers a few years back, including "Crazy Train". It's both the best and worst album ever made.).