If your line breaks, you lose the fish. For that reason, it is the single most important piece of equipment you use. Tournament bass fishermen who depend on landing fish for their income pay careful attention to their line. Many of them put on new line every day since landing one fish may make thousands of dollars difference in their check at the end of the tournament.
You can improve your chances of landing that fish of a lifetime if you use quality line and change it regularly. All of the major manufacturers make good line, try them all, pick your favorite and stick with it. When the line gets old, it gets weak. Keep up with the date you put it on your reel, and change it before it gets too weak to hold a trophy you hook. If the line starts to hold a curl after you cast, change it. It will fish better, and if it is curling it needs changing.
There is a line for almost any fishing condition you face and you usually need several. For bass fishing, you need a thin, clear line for fishing clear lakes. If you go upriver to stained water in that same lake and flip a jig in blowdowns, you need a heavy, tough line and it can be more visible. For crankbaits and spinnerbaits, one of the newer lines with little stretch might be your best bet.
Walleye fishermen are going to choose a very different line from muskie fishermen. For stripers, a line with some stretch might be better than a less forgiving line. Trout fishermen need invisible, light leaders but catfish don't care if you use a line that looks like a neon arrow pointing to the bait.
No matter what line you choose, pick a good knot, learn to tie it correctly and tie it carefully every time. I started using the improved clinch knot about 30 years ago and still use it today. I have never lost a fish because it failed. The Palomar is a popular knot that works well, especially with the new braided lines. You might need a drop of super glue on them. Many lines have knot tying instructions that work best with it in the pack. That might be the one to use.
Get a quality line. Tie a strong knot. And let me know when you land the biggest fish you ever caught. Contact me at email@example.com. Good fishing.