In the long term, I have found Breschiani's 80/20 and 75/25 cotton and wool blends to be the most durable. The 100% stuff of both wool and cotton are good...but not as durable...but they feel nicer
A Suitable Wardrobe is my first stop: http://store.asuitablewardrobe.net/hosiery.aspx
www.customshirt1.com (good selection of Marcoliani socks also)
www.howardyount.com (Marcoliani socks)
Side note: I tried Viccel. Nice colors for their wools...but they...