The softest merino shirt I’ve tried is the Outlier Ultrafine T-Shirt. The fiber used in this shirt feels almost buttery to the touch. It has a excellent drape too, due to the use of 17.5 micro merino fibers.
Merino has the ability to draw moisture into the very core of the fiber. This pulls moisture away from your skin. It also allows the fabric to keep breathing if it gets wet. As the temperature cools the natural crimps in the fabric insulate you.
Outlier constructs their shirts in Brooklyn, using merino from New Zealand. The company says this about the wool:
We use a Mackenzie 17.5, 195gsm Merino Jersey who’s name stems from the Mackenzie Basin on New Zealand’s South Island, where the bulk (but not all) of the merino used is sourced. It’s all non-mulesed, farm traceable and certified by AsureQuality. More importantly though the merino is a tight spectrum, low variance, top capped fiber blend. Generally merino micron numbers are generated by averaging a mix of various micron fibers. This means a standard 17.5 micron fabric will actually include much larger fibers in the mix. The Mackenzie blend is tight spectrum and low variance to create a more consistent and precise yarn. More importantly the Mackenzie 17.5 is top capped at 17.5 microns, there are no larger fibers making it a softer and smoother fabric, and in fact the yarns and fabric actually average a number lower than 17.5 micron.
Link: Outlier Ultrafine T-Shirt ($95)