When we had just barely started in the late 1990's, my very first manager had an uncle who was a head honcho at one of the largest sock companies in the world.  I met the uncle who had been giving socks to the missionaries where he lives for years, and explained to him the specifications of what we needed in a sock.
 
He took his expertise and combined it with my suggestions to make a line of socks exclusively for MissionaryMall that are made only for LDS Missionaries.
 
The Walk Sock sock is the most popular missionary sock.  It features 75% cotton, 21% nylon, 2% spandex and 2% rubber.  The actual construction for the sock is based on a tennis sock, which had rubber infused into the top of the sock so it stayed up on the leg, a mesh top on the footbed for breathability, and a padded bottom for shock absorption.  Also, due to the mesh top, this is by far the best sock for those hot and/or humid climates because it breathes extremely well.   

This blend wicks moisture from the foot, provides super durability and comfort, and the spandex makes sure that it doesn't stretch out and sag.  The sock is mid calf, which a lot of missionaries prefer-- long enough so you aren't showing your hairy legs when you cross your legs, but short enough so that they don't look like grandpa socks.  The toe seam is about as smooth as you can get it without buying a $30 pair of hand linked socks.  In other words, the toe seam is very smooth and it doesn't use a huge bulky seam that irritates some people.  

A few short stories about the walk sock.

We had a sales rep come in to talk to us about carrying his sock line, which is an excellent nationwide brand of sock that are big with hikers and outdoorsmen (Thorlo).  He wanted to see our current sock and when he saw it he said, "What do you charge for this sock?" Tyler, our store manager replied "$5.99"  The rep said "No, I don't want to know your wholesale cost, I mean what do you sell them for to customers?"  Tyler again said "We sell them to customers for $5.99"  It turns out that our Walk Sock was as good or better than the socks he was selling-- but his socks sell nationwide for about $12 to $14 per pair.

As the owner of MissionaryMall I get to try out a lot of samples of items we make for missionaries before we buy them.  When we first started developing these socks about five years ago, I got four pair of sample socks to try.  They are still my favorite socks, and it might sound strange to say, but every time I go to my sock drawer and a pair of walk socks is in the rotation, I choose them first and I am still wearing those original samples. 

We also have mission presidents that love the socks so much  that they end up buying them for each and every Elder in their missions as Christmas gifts.