The jack-of-all trades for many fly fishermen is intermediate fly line. They generally sink at 1 to 2 inches per second and can be used in a wide variety of conditions and if you could only use one this is a common choice both from the boat and from shore.
Keep in mind floating and faster sinking fly lines exist for a reason; fishing poppers on an intermediate can be done but it's frustrating, and if you need to get deep to get to the fish an intermediate takes a LONG time to get there if it ever does.
