Through the year I am continuously changing fly lines on my rods to match the conditions I will be fishing. It can be confusing with so many different lines on the market, so let's break down the things to look for into categories to help decide what is the best line for you.
The first and most important item, would be the fly rod. Fly rods today are made with very different tapers within each weight. Some are stiff or what is termed "Fast" and some are softer or "slower" tapers. The taper of the fly rod, in my opinion, is the biggest factor in choosing the line. Some of today's fast fly rods need an entire line weight heavier to cast well. In addition to whole sizes, many manufacturer's are offering lines which are weighted a half size more. All of this adds up to a lot of choices to help you dial in the best line for your rod by changing how deep the rod bends to load the rod for your cast and, in effect, changing the timing of the cast. To determine the perfect line for you and the fly rod you have is to get out and test the rod with various lines as timing of casting is different from person to person. Some fly shops may allow you to try different lines with your rod, and some may not, but lines are cheap and having the right weight line can make all the difference in getting the fly where it needs to be.
Next, the line being using should match the conditions I am going to fish. If I am in a tropical environment, I will want a tropical line and if I am in a cold environment I will want a line built for that (if it's not stated as "Tropical" then it is designed for cold water which is about anywhere North of North Carolina in the middle of the Summer). If you try to take a stiff core tropic line to cold water you will find a very knotty uncooperative fly line. Similarly, if you do the same with a cold water line in the tropics it will feel like you are casting a wet noodle and loop control/accuracy will be lost.
Lastly, it is good to determine the sink rate you need to get the fly to where the fish is and keep in contact with the fly. For 99% of fishing, a weight forward floating line will do the trick, and should be the go-to line before considering another line, but sometimes when fishing salt water you will need to get the fly down to the fish's level and keep in contact with the fly through rough/wavy conditions or use the dense line to cut through windy conditions. Just getting out and practicing in windy/wavy conditions will let you know over time when to swap out the line for a sinker. I personally like to use a sink tip for winds blowing over 20mph and a type 4 for rough and wavy conditions, but everyone is different in preference.
A lot of lines on the market today are very good in doing what they are designed to do making for some great days on the water. Hopefully, with the top points in mind, it will make purchasing the perfect line a little easier.
