If you’re looking for the best down jackets for backpacking, the first thing you should know is that real goose down is not always the best choice. Though it can sometimes be a good option if it's extra cold outside or if the jacket has an exceptionally slim design, generally speaking, down is not ideal, as it can be heavier and stays wet longer.
Unlike with regular camping, you have to carry your belongings when you're backpacking, so it's important for your jacket to be as lightweight as possible, as well as small and packable. Not only that, but the material also needs to be quick-drying so you can weather the elements and still be dry when it's time to hit the trail again. That said, real down jackets do tend to be a lot warmer, so they can sometimes make sense for cold-weather backpacking or wearing inside a lower-profile jacket.
Another important feature to consider is the exterior material. Because you're constantly brushing up against trees, foliage, and other elements, it's critical for the fabric to be tough and durable (preferably with a blend of ripstop nylon or polyester). You'll also want it to be coated with some sort of DWR or other water-resistant treatment.
With all of that in mind, take a look at this list of the best down jackets for backpacking. I've included a mix of real and synthetic options, so you can find the one that's best for you.