Back 2004, Cortland came out with a neat line called 444 Classic Sylk. These are modern synthetic fly lines that are made to imitate the old silk fly lines, without all the care & maintenance. They're smaller in diameter & more supple, just like a real silk line. Many anglers rave about these lines. The 444 Sylk line would be ideal for fiberglass &/or bamboo fly rods.
However, if you want the real thing, a true silk fly line is really neat to use. Of course they require more care than a synthetic line, but casting a silk fly line is like using a time machine. The best silk lines are made by hand from true craftsmen. They are considerably more money but, when you figure that a well cared for silk line can last decades, think of all the money you'll save in the long run. A great place to shop for silk fly lines is HERE.
For those of you on a budget, or are just getting yourself or a youth started in the sport, I don't think you can go wrong with the Air Cel Series fly lines by Scientific Anglers. They have been making these lines for many years. They are functional, dependable, & you can't beat the price. With the proper care, these lines will last you a few years. Plus they cast & behave exactly as they should. They are a true staple among fly lines & should serve any angler well.