Sierra Tower offers the best location and views in my opinion. It is the closest to the parks. One side of the tower faces the Neverland Pool area and the other side faces DTD. You should be able to see the Fireworks from many rooms on the DTD side. Most rooms in the Sierra Tower are considered view room and therefore cost more. The Marina Tower offers view room facing the Neverland pool. The other side faces the front parking lot, but also offer partial views of DL. I could see the fireworks from a standard, low-floor room in the Marina Tower. The Bonita Tower is the farthest from the park. Again, one side faces towards the Neverland pool, although the pool is not directly in front of this tower. Instead, there are koi ponds and pleasant little water falls (at least there were the last time I was there; did I hear they were remodeling this area?) I believe you can even see the fireworks from some of the high floor rooms on this side of the Bonita Tower, but from most rooms, the view of DL fireworks would be at least partially blocked by the Sierra Tower. The other side of the Bonita Tower is the least interesting. It looks towards the Paradise Pier Hotel and parking areas.
If you have a view room and want fireworks view, I would request Sierra Tower, fireworks view. If you have a standard view, you can probably request Marina Tower, Fireworks view. Have fun!