The most important job socks do is to prevent blisters. I suffer from a skin condition that makes my skin blister easily, so I have looked carefully at socks and tried a wide variety of different types.
1 The unholy trinity of blisters
Main article: Blister Prevention
Three factors work together to cause blisters; heat, moisture and friction. Socks don't do much to reduce heat, but they do reduce friction and moisture.
Contents
2 Material
Don't use cotton socks - cotton is one of the worst materials that you can choose for socks (or any athletic wear). I generally use Cool Max, which is a synthetic Fiber that uses capillary action to spread out sweat so it evaporates better. There are many other synthetic materials on the market that also work well. Some people get on well with wool; I use wool socks for hiking in cool weather, but not for running. If you choose wool, get high quality merino Wool, such as 'smart wool'. I've tried Bamboo socks; they feel nicer than Cool Max when dry, but hold more water and dry more slowly. Some socks include silver as an antibacterial material, which helps reduce odor. This is great in socks where you can't wash them for days at a time (long backpacking trips), but for running it is not a big deal. If you have a problem with shoe odor, then silver based socks may help.
3 Construction
Modern construction techniques allow for seamless, or near seamless construction, which is useful, as seams often cause blisters. These construction techniques also allow for variable thickness socks; I've not found much benefit from this approach.
4 Weight
If you are wearing light weight racing Shoes for a short distance race, then go for light weight socks. Otherwise, I would not worry too much about sock weight. It's worth remembering however that's the most important issue with blisters is having Shoes that fit correctly. Slightly different thicknesses of socks can allow you to tweak the fit of the shoe.
5 Compression Socks
I have found compression sleeves to be very effective, as our compression tights. However compression on the foot itself can cause problems. Compressing the bones in the front of the foot (metatarsals) can put pressure on the nerves and cause a Mortons Neuroma.
6 A comparison of socks
Here is my rating of various socks and sock combinations from best to worst, with a rating out of five.
6.1 Injinji Lightweight + Cool Max Wrightsock (5/5)
My favorite combination is to wear the Injinji lightweight sock with the thinnest Cool Max Wrightsock. This gives me toe protection and three layers of material. I find that this pairing is much better with blisters than any other option I've tried. I found that this combination works under the most extreme conditions of heat and humidity. The combination is also remarkably cool, much colder than you would expect for two socks with three layers of material. In cold weather I will sometimes use a slightly thicker Wrightsock in slightly bigger Shoes, but are more likely to simply use Vaseline for insulation. (Wrightsock at Amazon.com and Injinji Leightweight at Amazon.com.)
6.2 Drymax (4/5)
Main article: Dry Max Socks
I really wanted the socks to be even better than my favorite combination, and while they were better than most other socks I've tried, they were not as good. The Drymax socks did not keep my feet any drier than the Injinji/Wrightsock combo, and the friction from all of the Drymax socks I tried was higher. The Injinji/Wrightsock combo also gave me the toe protection of the Injinji socks which is a distinct advantage over the Drymax socks. I did try a Injinji/Drymax combo, but this was less effective than the Injinji/Wrightsock combo. (Drymax Maximum Protection socks at Amazon.com.)
6.3 Injinji Original Weight(3/5)
If you ever have a problem with blisters on your toes, try Injinji socks. They have individual sections for each toe, rather like a glove for your feet instead of a mitten. They take a little getting used to as your toes are not used to being separated. Having adapted to Injinji, I really like them. They make both a normal thickness sock and a much thinner liner. As noted above, the liner sock is thin enough to allow them to be worn with other socks as well. The Injinji socks are well-made using a seamless construction and technical material. The added toe protection gives them an advantage over the standard 'vanilla' technical Socks. (Injinji Original Weight at Amazon.com.)
6.4 Double layer socks (3/5)
The idea of double layer socks is that the extra layer of material reduces friction. The inner layer rubs on the outer layer, rather than rub on the skin of the foot. I like the Wrightsock dual layer socks and I generally wear their thinnest Cool Max sock, though I also have some of their thicker Silver based socks. Overall, these double layer socks have a distinct advantage over the standard 'vanilla' technical Socks. (Wrightsock at Amazon.com.)
6.5 ’Vanilla’ Technical Socks (2/5)
I have tried many different types of technical Socks, including Thorlo CoolMax, Asics Kayano, Feetures Light, Feetures Original, Smartwool, Icebreaker, and many others in the search for the perfect sock. The socks are very in some of the features they provide but function fairly similarly. Any sock that is made with a good technical material (and I include Merlino wool), but fits well and is seamless should work under most conditions for most people.
6.6 Waterproof Socks (1/5)
Waterproof socks, such as Sealskinz seem like a great choice for running in wet conditions. They are waterproof and claim to be breathable, so they should keep your feet dry. In my experience, the reality does not match up. The socks are waterproof, but rain will still leak in from the top a little. Worse, while the socks are breathable, they cannot cope with the amount of sweat that is normally produced. (The soles of your feet have a high concentration of sweat glands.) This problem is compounded by the way breathable materials are far less breathable when the outside is wet, which is the conditions they are normally used under. The fit of the socks tends to be poor, as they are not flexible like a regular fabric sock.
6.7 Cotton Socks (0/5)
Cotton socks are great for keeping your feet warm while you sit by the fire, but a terrible for running.