Sock yarn is my absolute favorite. So many colors, textures, and fiber contents - I've tried a little bit of everything.
I prefer to use self-striping sock yarn or solid yarns paired with tonal color blends. I only use the washable yarns (who really wants to hand wash lots of socks?!?) and the selection is constantly expanding.
Sock Yarn Sources:
Knitpicks has great sock yarns at reasonable prices - they knit up well, wear well, and wash easily!
Michaels has their own brand of sock yarn I'm trying out - Loops and Threads Luxury (not in all stores)
Sock Projects:
Elizabeth Socks with blue. teal and purple colorwork on a gray-blue background