Rio Fluoroflex Tippet is my first choice when fishing sinking flies. Fluorocarbon sinks and is abrasion resistant. To set up my favorite trout rig for trout I use 3x,4x and 5x Rio Fluoroflex Plus. Why buy Fluorocarbon:
These all make fluoro a great choice when fishing nymphs or dry fly fishing on lakes when you want to hide your tippet just below the surface film. The fish are very keen to dimples on the surface film and a tippet denting it (and refracting light) can be an easy tip off to a wary fish.