Something with both flash and movement. I use various "Flash Flies" on #4 or #6 salt water hooks with hot pink or blue hackled ones with silver holographic Estaz body and silver Flashabou wing and tail. The other fly I use at lot for silvers is a "fuschia bunny leech" tied on Alec Jackson Spey Hook #5 with blue Krystal Flash flowing from the head back to almost the end of the tail on each side of the fly. The "Flash Fly" tied with a wing composed of 1/2 silver and 1/2 hot pink of fuschia Flashabou, silver body, and hot pink hackle is also worth a try.
I fish them on floating, type 2, type 3, or maybe a type 6 sink tip, depending on water depth and speed. This is fishing where I also will almost always forgo the long belly lines I like and use a Windcutter simply because silvers like to chase a fly and the short belly lines let you retrieve the fly accross current for a greater distance.
If the silvers are sort of dour, I will use what I call a "Blue & Purple Pheasant" it has an oval silver tip, purple braided mylar body ribbing with oval silver, light powder blue calftail wing (you might have to dye the calftail to get the color because it is hard to find), and a blue phase ringneck pheasant rump feather hackle. I tie it exclusively on a #6 salmon iron.