I think that to define a master we have to look at four important things. First, we need to look at weather the patrons are paying ridiculous amounts of money with no evidence that the art will be great. Second, groupies. How big is their core audience. What are these fans willing to do? And lets not forget, minions. One of the biggest things that "masters" get that the regular artists don't get is a loyal following of minions. Last but not least, do we worship them? The work of masters sticks to cultures and is viewed with reverence years, decades, and centuries after its creation.
Masters today? we need to understand that art then encompasses many disciplines of today. Film, advertising/design, architecture, "fine" art, and others.
Film masters: Steven Spielberg, George Lucas, Alfred Hitchcock and many others.
Advertising and Graphic Design: Paul Rand, Saul Bass and others.
Architecture: Frank lloyd Wright and others
All of these people had and have patrons willing to pay big money for the master's brand. (the patron for a film in this case is the distributor, not moviegoers.) They all had and have a core fanbase willing to do anything just to be in their presence. They have minions who help them create the masterpieces. And their work is abiding in and some would say defining our culture just as did the work of the masters of the renaissance.