Icebreaker Superfine 190 Tech T

I have several of these and they are my "go-to" tops for summer hiking, trail running and other activities. They keep me just as cool as synthetic t-shirts, dry just about as fast and, unlike synthetic tees, don't hold onto odor after sweating in them or feel clammy when wet. And they are soft and don't itch at all. I also like the fact that wool is a renewable, natural material.

The only down side is that they are expensive if you pay full price--looking for clearance sales is a very good idea.