I was liking Smartwool or some poly-pro blends for years and I still do for winter. But I tried out Ray Jardine's concept of cheap, thin, nylon socks - and now that is what I use in the summer. I never hit the trail in shoes that aren't a great fit and I use low-cut hikers that are basically running shoes with a bit more stiffness in the sole. So the thin sock is all the padding / friction protection I need. They wash and dry really quick and also dry fast as you wear them if you walk through a stream. And, of course, they weigh less (on your feet, in your pack) than anything but going sock less.
For a UL thru hike, I'd have 1 pair of thin nylon and include one more pair in a drop box every month (they're cheap). Plus a Smartwool pair with some thickness to it. I'd use the wool socks if I got into spring snow, for whatever reason I needed a little more warmth, and for buggy areas. Mosquitos can get through thin socks easily and I HATE bites on my ankles. Also, the thicker socks would be used for sleeping on cooler nights.
Multiple-purposing: A cold morning start or cold hands in a cold wind can be addressed by using those wool socks as gloves. Looks goofy, but it works and it adds no more weight.
Edited to add: I've had durability issues with the Smartwool socks like Tim mentions. I wouldn't plan on doing hundreds of miles in them without backup. The thin nylon socks actually last longer, and I'd use the Smartwool for relatively few miles.