NH Cavallieri Hotel
This is a fantastic building opposite the railway station, which has 100 rooms. It is a good choice if your flight arrives in Pisa at night because you can catch the bus at the airport, and within 15 minutes, you can be right in the hotel. The common areas are modernized and well decorated, but the rooms are not. Breakfast is more than fair, croissants are fantastic though, and the walk to the tower is just over 20 minutes. I recommend a hotel on the other side of the river if you are going to stay more than one night. The ambient of the city is different once you cross the bridge.