I fish a dry/dropper rig a lot.
In the early Spring, I'm usually using an Elk Hair Caddis #14 with either a #16 or #18 Prince or a red Copper John off the end; about 12-18" off the bend in the dry. I also use fluoro for the dropper; usually in a 6x. I've also used Royal Wulffs as the dry; both the Coachman version and the White version. Both work almost as well as the Elk Hair Caddis if you're in a stream with a reasonable amount of current. When the current really picks up, I'm back almost exclusively with the Elk Hair Caddis. There's something about the hollow chambers in elk hair that just makes that fly float in almost every stream situation that you're apt to encounter. I'll even use an Elk Hair Caddis dry with a dropper when I'm casting into the base of a plunge pool; knowing that even though the dry will be sucked under by the plunge currents, it will re-emerge downstream with the dropper still intact. I've actually had good strikes on the dry caddis when it's underwater! Which, as you might imagine, has given me an idea or two.
In the Summer, I'm usually going with a #12 Chernobyl Ant as the dry and hanging amost anything off the back as a dropper. I'll often times fish a soft hackle off the ant; sometimes a tandem double soft hackle dropper.
A couple of weeks ago, I was fishing a triple wet fly rig off an intermediate sinking tip line on a couple of the limestone creeks in Central PA. It's a nice rig. You fish it across on the diagonal, mend and let it dead drift downstream with something akin to a Leisenring lift at the end. That rig gives the fish 3 different choices and once you find out what they're striking on, you can go to a single wet set-up if that's what you prefer. I don't recommend that you do much false casting with a triple fly set-up; not unless you really relish untangling a knotted leader!
Another rig that I like to fish at plunge pools is a double nymph. I use a floating line with about 3 ft of intermediate sinking line joined to the tip and follow that with a leader with something like a #14 beadhead pheasant tail then a #16 red Copper John (yes, I like that fly a lot - and so do the fish!) tied off the bend in the first nymph. I fish it up and across, hold the rod high and retrive with an almost vertical motion.
Frankly, now that I think more about it, it's been a long time since I've put a single fly on my line if I'm fishing for trout; unless there's a good hatch going on!