That's impossible. Manure and Real Madrid have the most "Fans" (quote unquote) in the world. However, what Benfica does have is the highest amount of "club members," as they were the first to offer club memberships for fans. Apparently they have like 500,000 club members.
Who the hell knows. Just because you won three straight Champions League plus one doesn't mean you're a better side than Juventus throughout history, as you also have to take into account competition. Bayern has 20 German League titles (I think) while we have 27, not even mentioning that Italy is by far the more superior league. Europe is not everything, you know.
And to be honest, I don't really care who the better side is over history. Titles are not everything in football.