In my mind the mural capital of the US is Philly, but according to this video, Los Angeles was the city that held this title some time ago.

Oliver Riley-Smith made this short video documentary about how mural artists and graffiti artists are battling each other in neighborhoods where murals are being painted over because the California Redevelopment Association (CRA) deems graffiti an eyesore and instead of rehabilitating existing murals, they are just painting them over entirely.