Subjective, I know, but 4:3 was a lot more pleasant to work with, at least for me. 16:(9|10) might match movie aspect ratios better and thus would be better for watching them, but I do other things more often than watching movies on my computer screens (it's different for a TV, though).

Conversely, 16x(9|10) allows me to put 2x portrait documents next to one another comfortably. Alternatively, I can work on a document with a 2-page view.