Tyvek – I had a tyvek long sleeve shirt on a backpacking trip last month and it was amazing to watch the mosquitoes land on it and be so confused. I don't know if it's the dense fibers or the color, but they would just fly away after a second or two. Also made for a light (3oz) wind / water layer. For a base layer, since that's your original question, I swear by icebreaker wool shirts, loose fit so air can circulate…
