I’ve been testing wool clothing in my lab for about ten years. I’ve tested almost all the major brands of merino wool sweatpants. These are the softest ones I’ve found:
100% Merino Wool Warm Up Sweatpant by Woolly
The “pro-knit” fabric is an 18 micron merino that is “woven into a compact interlock structure”. The result is a very soft and lightweight wool pant that also seems quite strong.
The wool used is 18.5μ merino (grade: very fine). The fabric weighs 250 g/m2.
95% Merino Wool Sweatpants by Woolly
These pants are thicker than the “Warm Up” pant above — fabric weight is 320 g/m2 and the knit is different (but soft).
The wool itself is the same fineness — it’s 18.5μ merino wool from Australia. It also contains 5% elastane.
Icebreaker Shifter Pants
Icebreaker makes these “Shifter” sweatpants, which are also very soft.
These pants are made with 98% Merino Wool, 2% Lycra. The merino is 260 g/m2 (mid-weight).