Among my fave prog bands are the usual Yes, Rush, King Crimson, Floyd, Genesis, early Kansas, ELP, Gentle Giant and some Dream Theatre. Tull is probably most important to me, mostly for sentimental reason. After I listen to these bands, I have a hard time accepting other prog bands because I don't hear anything better than these bands. Mars Volta doesn't quite grab me, but maybe I'm not getting them. Spock's Beard, same.
However, I'm open to listening to bands I've misjudged or overlooked. Who do you like?