I cannot comment on Cipriani and Amman, as I have not been to them, but I LOVED the Gritti Palace, and I would recommend it in a heartbeat.

Things that I did like a lot :
- Location: the hotel is located along (the start of) the Canal Grande, facing the Basalica di Santa Maria, only a 5 minute walk from the Piazza San Marco.
- The floating terrace, right one the Canal Grande, is the location where breakfast is served and where you can relax after a day of sightseeing. It was really so mesmerizing to observe the life on the Canal Grande. An added bonus is that the terrace is facing the sun all day long.
- the décor of the hotel, with all its antiques, is overwhelming but in a very tasteful manner. If you prefer a trendy hotel, look somewhere else (but frankly, why would you want that in a city like Venice?).
- the staff was without doubt the most friendly staff that I ever met at a hotel. They treated us like royalty. Kudos to all of them.
- As a platinum SPG member, I was upgraded to a fantastic suite, again featuring old style furniture, but with all the modern amenities one could imagine (the hotel was recently completely renovated). The bed was the best hotel bed that I ever slept in.

Things that I did not like that much (& suggestions for the owners) :
- the hotel is very, very expensive. While I can understand the room rate (they are Venice most exclusive hotel), I did find their prices for food & beverages beyond reasonable (starters around 30 euros ; main courses around 60 euros ; a coke 10 euros). Venice is very expensive, but nevertheless, we still had excellent 3 course dinners in Venice at some highly Michelin recommended restaurant for 60 euros per person. It is a pity, as the high price of the Gritti Palace restaurant put me of trying it out.
- while breakfast (buffet) was excellent, I thought that for a hotel of this caliber more choices would have been offered.
- since the hotel reopened, a rooftop has been added to its portfolio. We were given a tour. The very large rooftop (with hottube) is beyond words, and a unique feature in Venice. Unfortunately, it is a private rooftop, belonging to their most expensive suite (so you will only get it when you book that particular suite). This is where I do not understand the hotel’s decision very well: not only is the suite that own the rooftop totally underwhelming & dark (with very small windows), but why they did not decide to open up that amazing space to all their hotel guests is beyond my comprehension (as it would add so much more to the experience of all their guests, and it would make this hotel unbeatable in terms of competition as no other hotel in Venice has this kind of space).

Enjoy Venice!