I would think about five properties:

Hilton
Gewandhaus
Steigenberger
Taschenberg
Westin

The first four are in five minutes walking distance from each other (calculatated from the Hilton), the first three play in the same leage pricewise. The Westin is on the Neustadt side of the river and is the only hotel that offers the romantic view over the skyline. It depends a bit how much you want to spend for your specific room and whether or not you have got status with one or the other chain.

If no status is relevant: The Taschenberg is generally the best hotel, all the others are more or less the same, you get a nice hotel room unless you want to spend extra money for a view. Steigenberger offers unspoiled Frauenkirche view, Hilton a less good Frauenkirche view. Westin offers the skyline, Taschenberg the Semperoper and Gewandhaus offers not really a lot of a view.

Without status I would probably chose the Steigenberger, with Starwood Platinum the Westin (you get massive suites).