Here's my vote:

1. Steigenberger Graf Zeppelin
2. Hotel am Schlossgarten

Both are located opposite the main train station and are close to parks, parliament and shopping areas (within walking distance). Both feature a one-star michelin chef at their signature restaurants (No.1 Italian and No.2 French cuisine).

3. Le Meridien

Along a busy freeway and opposite of one of Stuttgart's largest parks (the Schlossgarten). Used to be an IC. Renovated rooms.

4. Millenium Hotel

As mentioned by MegatopLover (excellent description btw ^ ) it's located in the outskirts of Stuttgart close to the Interstate highway Stuttgart-Munich and the airport. It in the middle of an entertainment complex (casino, musicals, cinemas, restaurants, spa, pools) and has a good connection to the city by car and local train.

5. Maritim

In the middle of the city near an entertainment complex (bars, restaurants, cinemas, music hall). You can walk to main points of interest in the inner city.

The current Moevenpick (on-site airport hotel) is quite good, a second one will be finished later this year next door for the opening of the new exhibition complex at the airport. Regular local train service to the city.

If you are looking for something special in the middle of town, try this one:

http://www.zauberlehrling.de

But beware, not one of these hotels compare to a RC or FS or Shang or MO or...

The hidden gems are (way) away from town...