I use it all depending on the conditions and the trip. Everything from my pretty light marmot precip jacket as a shell or rain coat, to my Fjallraven waxed cotton jacket and pants, underarmour, wool, fleece, down, it all just depends on what I'm sort of expecting. Cool damp temps, then its wool all over, but dry, cold and high wind, its all synthetic and down. Sometimes its wide temp swings and there will be limited likelyhood of using a fire, then I'll pack the synthetics, but when I know I'll be having fires, then the cotton and wool are what I look to. There is no one good answer. For me my temps range from right around freezing to 35C with full sun and high wind. So keep that in mind. For my conditions, a light wool shirt and pants is a better insulation layer than a bulky fleece in most cases, but the fleece is the easier layer. I also have more specialized stuff for cold weather paddling, (cold, as 10C)
I would like to have a one-and-done solution, but the reality is that between where I'm conditioned at, what the humidity and activity level will be, (or type of activity) I have to have a few options to meet the conditions. Everything is a compromise. But the synthetic fills like climashield are getting good enough that at least the synthetic vs down question has some easier answers. Often for me its finding a balance between night time insulation and high day-time temps, along with bug and UV protection.