The biggest surprise was how light the shirts felt even when wet and how quickly they dried after we stopped running. Some shirts are thicker than others. You'll want to choose a shirt based on the conditions where you live and how easily you become chilled while running. Obviously, the heavier the underlayer, the warmer you'll be. But remember, you can become too warm running in a heavy base layer.
Most lightweight shirts are sufficient for most runners in moderate winter conditions.
Your shirt should fit snugly for maximum moisture management and warmth. If there's air space between you and your first layer, sweat will stay on your skin, cold air will find its way in, and you'll shiver your timbers. Following are evaluations of the 12 shirts Bob and I tried. This medium-weight shirt feels soft and cottony. The fibers pick up moisture from your skin, and microfiber filaments transport the moisture to the surface for quick drying.