The single most important thing to consider is that the fly line must match the weight on the rod it is to be used on. Due to the many manufactures and wide range of fishing conditions there is an almost endless selection of tapers and variations.
Fly lines basically come in level, double, weight forward and shooting tapers. I am sure everyone has their opinion on this matter, as to what is the best overall line. You will have to decide which taper is best suitable for your fishing conditions. Fly lines come in ether a floating or sinking or a combination of the two.
Weight Forward -This is by far the most popular line and used by most people that I go fishing with. As the name suggest the weight is in the forward section of the line. The weight of the line is usually measured in the first 30 feet. Generally this line will permit longer casting distances. This taper is an excellent choice for windy conditions or long distance casting. If you are fishing in tight casting situations be aware that this line is hard to roll cast.
Double Taper - These lines have a 6 foot to 10 foot tapered section at each end of the fly line and have a long level section in the middle of the line. This line will give you a more delicate presentation of your fly. Not a very good choice for windy conditions or long distance casting but an excellent choice for roll casting. A double taper line can be reversed, making this an economical choice as you can use both ends.
Shooting Tapers ?These lines while they are great for making very long casts, lack the delicacy needed for general all-around fly fishing. The line, due to the weight in the front of the line, can spook the fish when it hits the water. This line is harder to control for a beginner. Also since the back end of the fly line is thinner than standard fly line, it has a wicked tendency to coil and get knotted up. For beginner anglers, a shooting taper fly line is not recommended. A shooting taper fly line works well to make very long distance casts and fish in windy conditions.
Level Taper ?A level taper fly line has absolutely no taper and is of uniform weight and width for its total length. While theses lines float extremely well they are the most difficult to cast and handle. Beginner anglers should stay away for this taper of line.
The best overall fly line for a beginner would be the weight forward, as you gain experience you can try the different tapers and see what feels best to you.
