This is the question coming up frequently and I am sure you will get at least five answers: Danieli, Gritti Palace, E&R, des Bains and Westin Excelsior.
It it very personal and experiences differ. I would never ever go to any other place but the Gritti or the Danieli but others would not even think of it. Try a search, there is plenty of good stuff on Venice on the Starwood Forum but I am pretty sure at the end of the day you will five answers in your head.
By the way: Cipriani is not "outside" of Venice but an hour drive to Venice. If is a nice place if you love upmarket country hotels but for a visit to Venice, no....