There are lots of brands, styles and types of fly fishing leaders on the market today, and as long as you choose a good quality one that is suitable for your type of fly fishing and one that provides a good value for its price you are going to be just fine. The most common leader types today are knotless tapered leaders. The most common materials are solid nylon (Commonly called monofilament) and fluorocarbon (technically known as polyvinylidene fluoride, or PVDF, but let's just call it fluorocarbon from now on!). The term monofilament is a little misleading, because leaders made from both nylon and fluorocarbon can be monofilament (single filament), but for modern fly fishing we usually use nylon and monofilament synonymously.
Today's tapered leaders are designed according to formulas that allow the energy flowing from your rod through your tapered fly line into the leader to be transmitted smoothly to the end of the leader. Properly tapered leaders can mean the difference between a good cast and a tangled mess.