Dark green buzzer.
Another all-time classic. This time it's definitely a 12, none of that "maybe it's a 10, maybe it's a 12" crap. A wonderful fly this. Sometimes black's just just too dark for the fish; that's when they want green. A good fly to fish in April. A good stillwater set-up would be to put this on the bob, the black buzzer in the middle and a leaded tadpole or cased caddis on the point. Fish this slowly on an intermediate or floater (depending on where the fish are) and you have a great tactic. Later on in the season they'll also be on these buzzers, but those fish are best nailed with suspenders.
Take a size 12 hook, squash down the barb, dub some *dark green* seal (perhaps with a little light green mixed in, cos that's sexy), wind it on and rib it - good these flytying instructions aren't they?