Stone, because it sinks like one. And because it's a serviceable imitation of a stonefly nymph.
Simple, because even we can tie it.
No points for creativity here. This is a long shank nymph hook (Daiichi 1720 in this case, size 6), lead wrapped from bend to eye, rubber legs tied in here and there, and topped off with a few wraps of chenille. A simple, deadly pattern. This fly and others like it have been around since Lefty was in diapers. Which makes me wonder why it didn't become a permanent resident in my fly box sooner.
Speaking of deadly by the way, check out the EPIC carp fly swap that McTage is hosting over on Fly Carpin'. EPIC.