Well, I'd love to make this a poll but I don't really know how.
I'll start: So far, my favorite sock yarn is Lorna's Laces Shepherd Sock. But I've only ever knit with that, Austermann Step, and KP Stroll. I like that it seems to hold up better than the other 2. Austermann Step was too thin for my liking and fuzzed a lot, and Stroll didn't handle my repeated frogging too well and started to pill. To be fair I was making Jaywalkers and had to frog about 50 bazillion times (and still didn't get it right).
So I want to try some other sock yarns when I'm done with my stash of LL. I'm particularly interested in anything that wears well and can be machine washed (drip dry). Also I don't really like the kinds that stripe (like Step). I like the variegated ones or semisolids better!