I wear a lot of merino wool next to skin layers and have somewhat sensitive skin. I love the stuff! Wool is warm when it is cool, and cool when it is warm. Excellent SPF, even in thin fabric. Natural odor resistance. I use it for cross country skiing, gym, and travel.
Not all merino wool is created equal, as several have noted. The brands that specialize in non-itchiness use a particularly fine grade of wool from certain sheep breeds, and they process and spin a way to reduce the natural "barbs" in wool fibers. Brands like Icebreaker, Smart Wool, Ibex, Patagonia and SuperNatural all make washable, non-itchy products. My comfort level varies, even within the same brand/style. When I get a slightly scratchier shirt, I just call it "bad sheep" and take it back. You really have to be able to try it on, and return it if it isn't OK.
I have several Icebreaker and Smartwool zip hoodies. I love them. I have short and long sleeve shirts in all different weights ranging from 140 gsm (grams/square meter) to 320. Some brands add nylon to improved durability and wind resistance. I have lots of merino wool undies - I like the Siren Hipkini from Icebreaker for travel.
140-150 gsm is thin - light colors can be sheer. 200 is my go to to for a light weight base layer, and is usually substantial enough wear alone. 260 is like a light sweater, 320 is heavy sweater. My hoodies are around 260 gsm.
I machine wash delicate cold and hang dry. The Smart Wool labels often say you can dry them, but I usually don't.