I have stayed at the Steigenberger Graf Zeppelin before and found it a very good 4 - 5 star hotel. Public areas of hotel were quite dated (this was about 4 years ago so it may have been renovated since then) but the room was very modern and comfortable. There was even a cross tpye gym machine in the room.

There also used to be an Intercontinental in Stuttgart but from searching the net it seems to be no more. I suspect that it is now Le Meridien.