For starters, the REI Co-op Flash Jacket is a great jacket but it is not waterproof. It is treated with a durable water repellent (DWR) to shed light moisture but it would not be ideal for a waterproof shell over the long term or for the rigors of backpacking. The good news is that any jacket that is designed as a waterproof shell will also be windproof!
That being said, there are many options to choose from. Once you have narrowed it down to a couple of options I would think about the length of the jacket (if that's important to you) as well as other features like the hood (whether its adjustable or fits over a helmet) and the amount and location of pockets. Personally, I look for an adjustable hood and hand pockets that sit up above the hip belt when I'm wearing a backpack. The REI Co-op Drypoint GTX Jacket is a really good example of those features.
I hope this helps, best of luck with your search! Thank you!
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