Staying comfortable in the cold is less about one heavy coat and more about a system you can adjust on the move. Three layers do the work: a base, a mid, and a shell.
The base layer manages moisture against your skin — choose merino or synthetic, never cotton. The mid layer traps warmth; fleece or light down both work. The shell blocks wind and rain.
The key skill is venting before you sweat. Open zips and shed layers on the climb, then add them back at the summit before you cool down.