What excellent university DSP programs exist and what makes them excellent?
Firstly, lets assume you're talking about graduate programs. X being such a specialized program would only be available at that level of study. 

In a nutshell:


Large list of cross listed (undegrad/grad) or only grad courses in all areas of study. You still need to have courses that can advance your overall knowledge of engineering. 
Large variety of courses in your area of study. (If your studying DSP, its not enough to just take a DSP class, you may find it relevant to study data compression, machine learning, Free-space laser communications.) This also means you wouldn't have to complete a large number of independent study programs. 
Large Number of Faculty in your research area. If you are interested in studying music signal processing, you would want to do research with a professor who has experience in that field. If there are 10 professors, you could be more selective about who to work with or who will be the best fit for your research area. 


As for schools with decent program: Binghamton University, while it is not well known outside of New York, its master's programs is well structured, large number of faculty in EE at least 3-4 DSP professors, 10-20 graduate level courses offered a semester and large number of those are DSP related.