I’m not much for being out in really cold weather anymore, but when I do I really like my Buffalo Mountain Shirt.
Made by Buffalo Systems in the UK. They are made to be worn next to the skin, or only over a light tee-shirt. I have found them to be very warm in below freezing temperatures, and very versatile. With many ways to regulate your temperature.
I ordered mine from Jackson-Sports in Ireland. They do a great job with everything I have ordered from them. I think some suppliers in the USA. moosejaw, backcountry, etc. may carry them but are more limited in sizes, colors, etc.
Buffalo Systems also make a similar model called the Special 6, developed for British S.Forces, Mountain Rescue teams, etc.
BMS, is the original, but other companies such as Montane, make clones. They call their version the Montane Extreme Smock. I wavered for a while which one to order, but went with the original. One advantage with the Montane model is the hood is included. For the Buffalo Mountain Shirt the hood is an option that can be ordered separately. The Montane Extreme Jacket would be the competitor to the BMS Special 6.
Anyway, features not obvious in the above: the BMS has two-way side zippers on each side, they can be opened a little from top or bottom to regulate ventilation. The lower front pocket is open side to side, and inside it is a strap that can be tightened to shutoff drafts getting inside. The chest pocket is convenient to store gloves, mittens, hat, etc.
I also have the Buffalo Pertex Pile Mittens. Work great too.
I discovered Jackson-Sports when I was looking for a legal supplier of the Silva, Expedition 54 compass. (Silva Sweden compasses are no longer legally / officially sold in the USA. ) The discovery ended up costing me a lot more money after I discovered these items, well those and the Montane Terra or Super Terra pants. However, I have been quite pleased with all of them.