Appropriate socks help keep your feet at a comfortable temperature and play a large part in preventing blisters. Blisters come from the combination of friction and moisture. You want socks which wick moisture away from you feet and minimize the amount of friction between the sock and your foot which is often best accomplished by using a double sock or two socks. I think nylon and wool are the best materials for socks, and cotton is really terrible.
My Choice
I primarily use Darn Tough Ultralight No-Shows which have lasted >=4000 miles before needing to be replaced. and recently added in Xoskin anklet toes socks which seem to resist odor reasonably and are effectively preventing blisters between my toes. My first pair develop a hole that was easily repaired after 640 miles. At 800 there are some small holes in the highest friction area (between my hammer toe ) but they are still working well. On my forth pair… average 1200 days of use, way less than my darn tough dock but longer than a pair of trail runners but longer than most toe socks I have tried. This has been mostly wearing sandals… I expect they might last longer inside shoes. In cool and wet weather I will sometimes wear light socks combined with Rocky Gore-Tex over socks. In cold conditions I have found vapor barrier socks to be extremely useful.
Sock “Fit”
Keep in mind that there needs to be enough room in your shoe for your feet and the socks you select. Socks which are too thick for your shoes will crowd your feet and result in blisters just as quickly as your feet sliding around. I would strongly recommend trying a variety of different socks and figure out what works best for you. Socks are cheap and will often mean the difference between a miserable blister plagued trip, and one with comfortable feet.
Double Socks for Hiking
Conventional wisdom for people wearing hiking boots is to wear a heavy wool sock, with an very light weight inner liner. Dr. Murray Hamlet of the Army Cold Weather Research Center confirmed this with an extensive testing of a variety of sock systems. The first set of these tests was documented in the report Impact of Sock Systems on Frequency and Severity of Blister Injury in a Marine Recruit Population.
Hamlet and team found that a thin liner sock made of CoolMax, and a thicker sock made of wool&polypro (fuzzy side out) had a significantly reduced the frequency of blisters. This is because the slipping is happening between the socks rather than between the foot and the sock. TechSpun sells the socks designed by Hamlet. Besides TechSpun there are a variety of companies which sell high quality wool or wool blend backpacking socks including Bridgeport, Dahlgren, Darn Tough Socks, Fox River, SmartWool, Thorlo, Wigwam.
Lighter Socks
People carrying lighter loads and using trail runners or walking shoes often prefer socks that have less bulk. Since lighter shoes tend to flex with the foot, an inner / outer sock combination isn’t as critical as when using boots. Some people find double socks are helpful with trail runners. Some low bulk ways to get double socks include using a pair of knee high stockings + a light outer sock, two pairs of nylon dress socks, WrightSocks DoubleSocks, or X-Socks (I have no experience with these pricy socks).
Single socks I would recommend for trail runners include Darn Tough Vermont Running socks, xoskin socks, Injinji toe socks, Smartwool Adrenaline, asics running socks, Inov-8 Mudsoc (coolmax), and the Bridgedale X-Hale Multisport Socks (multi-fabric blend).
Extreme Conditions
I have a pair of posts which address footwear (which includes socks) in particularly wet or cold conditions.
Cheap
Use a double pair of nylon dress socks.
