A question we receive regularly, but not one simply answered. The best jacket to keep you warm depends on where you are, what the weather is up to and what you are up to. If enjoying the outdoors in dry, cold conditions we very much recommend a down insulating jacket as it breathes superbly (making it ideal for fast moving activity) and is able to effectively hold the heat your body generates close to you with ease. If you’re looking to be toasty and comfortable in very cold conditions look for high fill weight and fill power.
If you’re in cold and wet conditions, though it will never out perform a waterproof, a synthetic insulating jacket vastly out performs down when it comes to insulation when wet. Synthetic insulation such as Primaloft repels water and retains upwards of 90% of its insulative properties. When aiming for an as warm as possible synthetic option, thicker jackets with more insulation will work best at the cost of packability and breathability.