Previous suggestions are good for central located hotels.

I can add (5 and 4 stars, some with excelent restaurants) the Luna Baglioni, the Londra Palace, the Metropole, the Saturnia, the Monaco (ALL San Marco area), the Bonvecchiati (closer to Rialto), Ca' Pisani (Accademia).

Back to Austria, Velden am Woerthersee as well as Poertschach am Woerthersee have nice 4 stars hotels on the lake.