Google does not provide a list of sources for its News service, but this site runs a php script that captures the Google News home page every 15 minutes and then logs the news sources it finds. You can view sources for all countries or select an individual country from the drop down list. However, the country option does not appear to be very accurate so probably best to stick to the “all” option. As of April 5th, a total of 2990 sources were listed.
The default listing is by source, but you can change that to Frequency- that is by the number of articles per source. According to this site, the top 10 so far are:
El Universal (México)
New York Times