For trout fishing, using small flies, I use a tapered mono leader which is usually about a rod length long, when fishing in the film or swinging drys. If the fly is very large and /or I need to cast far, I can use a 10' Poly floating leader with 4'-6' tippet added, as needed.
For sink tip use, I add 3'-4' of Frog hair Mono, or Fluro if the fly is small, or in clear water, or if I want the extra depth to create a bit of droop, to get the fly below the sink tip and fishing a little deeper.
For nymphing or high sticking, I make knotted leaders using Flurocarbon, which also has enough stiffness in it to allow good turn over, using a 16 lb. butt section.