Whether you're buying your first fly reel or upgrading after many seasons on the water, you'll need to take some time to determine which fishing reel is best for you. First, consider the type of fishing you'll be doing most often. What is your target species? Will you by fishing in shallow water, or deep? Flowing or still? What weather conditions can you expect? If you'll be fishing in saltwater, make sure to purchase a saltwater reel that can stand up to the corrosion and provides a sealed drag system to keep the salt from the inner parts of the reel.
Once you know your target species and line weight, you'll need a reel to match. If you'll be fishing streamers, you'll want a larger reel than you otherwise would for the same size weight, which will help you gather up line to reel in. For saltwater species, you'll need a large reel for the larger fish, and you may want to err on the side of oversizing. Larger handles can also help for saltwater, which requires more reeling in than freshwater fly fishing.