Danieli
As Flying Lawyer said, you'll get five answers if you ask FTers. My #1 is Danieli, for the experience. Any of the rooms facing San Zaccaria are great. Breakfast on the roof, drinks in the lobby bar, and a very knowledgable concierge staff are all big pluses. Being able to walk out in the morning to the street merchants and take a quick right to San Marco squre is great as well.