That really depends on your budget - but if you're considering the new Hilton, you're thinking of pricey digs, so either the Gritti Palace or the Daniele, even the Bauer Grunewald. You'll be in the heart of city. Slightly less expensive options would be the Giorgione (landlocked but very good value) and Ca' Favretto - on the GC but further from S. Marco and thus less expensive.

As for the Hilton - I agree w/the previous post regarding the reality of it meeting the scheduled summer opening. I lived in Venice for 3 years, and experience further tells me, that even if the Hilton opens on time, their first season will be one devoted to working out problems, ranging from electrical to staff and of course, leaks/water problems of every conceivable source.