Under Armour Cold Gear. The shirt/pants sets that I have are 10 years old or so. I have some newer shirts and they are about the same. I use the Under Armour as my base layer and then regular long johns (shirt or bottom) over that if it's quite a bit colder out. This is all still relatively thin and I can layer my hunting, fishing, ATVing, or motorcycle riding gear over that.
There are "generic" and less expensive brands of Under Armour sold at Wal Mart and other stores, and according to some of my year-round motorcycle riding friends, these brands are as good as Under Armour at doing their job, but wear and tear a lot quicker.
On the subject of base layers, Under Armour HEAT gear works wonders on hot days. Long sleeve heat gear helps you to feel the breeze when out on the boat or other activities where you're on the move.