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 wear Injinji toe socks any time I am expecting to be walking or running for an extended distance because they prevent blisters between by toes. I like the wool models, but my toe-nails often put holes in them, so normally I use the coolmax models. Generally I like their light or original weights. These socks took some getting used to. The first several times I wore them I was very aware that the socks were on my feet and between my toes and I hated them… so I gave up on them. I had several friends kept saying how great they were. So I tried again. I wore them around town for a couple of week, and then pushed through a few days the trail. In the cool to cold weather I switch to Injinji liners combined with Darn Tough Vermont Treking Socks, Rocky Gore-Tex over socks or vapor barrier socks.
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.
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 two pairs of nylon dress socks, WrightSocks DoubleSocks, X-Socks (I have no experience with these pricy socks), or normal light running sock next to the foot with a liner on the outside.
Single socks I would recommend for trail runners include Darn Tough Vermont Running socks, Injinji toe socks (for people who have problems with blisters between toes), Smartwool Adrenaline, asics running socks, Inov-8 Mudsoc (coolmax), and the Bridgedale X-Hale Multisport Socks (multi-fabric blend).
I have a pair of posts which address footwear (which includes socks) in particulary wet or cold conditions.
Use a double pair of synthetic dress socks.