This obviously requires careful thought. I've already pondered the question for several hours, and, while I have any number of candidates for my top ten list, there are only three that I'm sure will make the final cut. (Although I'm very fond of Rockwell -- as you can see from my avatar, which I chose for sentimental reasons -- I don't think there will be any Rockwells on the final list.)
In no particular order, my first three choices are:
Vermeer -- Little Street
Sargent -- Ena and Betty, Daughters of Asher and Mrs. Wertheimer (I'm a big Sargent fan, so I had trouble narrowing my choice down to only one of his paintings. I finally chose this one because I can't imagine ever getting tired of Ena's and Betty's company.)
Domenico Veneziano -- The Annunciation (I've only seen this painting once, when I visited the Fitzwilliam Museum in Cambridge over 20 years ago, but the memory of it is still so vivid that it was one of the first paintings I thought of when I began making this list.)