I was fortunate to hear the talk by Dr Xavier Bray, from the National Gallery, who demonstrated to us the greatness of Velazquez, and the considerable talent of Zurbaran.
Some information about Dr Bray is at this link from the Threadneedle Art Prize website
Of course we have been able to see two magnificent shows of Velazquez in London, fairly recently. I have been lucky enought to visit to Prado in Madrid, too where there are fantastic paintings by him.
Here is useful information about Velazquez with some thumbnail images of some of his work.
Philip IV of Spain in Brown and Silver