The meat and potatoes of any well stocked fly box. Streamers and Bucktails account for a lot of big fish in both fresh and saltwater. Mostly they represent small baitfish and young gamefish, but they can also imitate leeches and worms. Some are imitative of a particular species while others obviously don't look like anything natural, these are considered "attractor patterns" such as the Mickey Finn. It's a good idea to have some of both in your arsenal, they all represent something good to eat in the eyes of larger fish, and the bottom line is big fish eat little fish!