I usually wear just one pair of socks, with the exact model depending on the season. For 3-season use I want a sock that features:
The best sock I have found is the DeFeet Wool-E-Ator. Even after long high-intensity trail runs, presumably during which my feet have been sweating a great deal, I find the sock to be dry. And because the sock is so lightweight, it takes a minimal amount of time for it to dry out after being completely soaked, like by a rainstorm or a ford.
For winter use I want a sock that:
The best sock I have found is the DeFeet Blaze (the wool version), the dense knit of which keeps my foot as warm as thicker socks do, but without the bulk.