Superlight, smooth 100 percent merino wool. It's cooler than a cotton t-shirt in hot weather, and in cold weather, of course, you're not going to be wearing a cotton t-shirt, right? Not if you exercise and sweat, at least. This is a super garment, and the practical layerist will soon find out that when she wears it over another garment, she can strip it off when she starts sweating or it gets too hot for her. This with a sleeveless or a long-sleeve wooly over it makes a comfy, useful, ready-for-almost anything combo.  Crew neck.
Sized small, so if you're ordinarily a medium, get a large. If you're normally a large, it'll be snug. Not bad, just snug.

Download this sizing chart.